fig1 shows the appearance of the first embodiment in which the present invention is applied to an electric driven screwdriver , which comprises an electric driven screwdriver main body 10 , a position sensor 20 and a control box 30 . the electric driven screwdriver main body 10 contains a power circuit , an electric motor , and so on . when a trigger switch 11 provided in a handle is turned on , the electric motor is driven and a chuck 12 at the end of a drive shaft is rotated . the initial direction of rotation of the motor is selected by switching a selector switch 13 for switching forward and reverse rotations . as shown in fig2 a box wrench 14 is detachably fixed to the chuck 12 so that a nut 104 engaged with a bearing bolt 102 vertically fixed on a foundation slab 101 can be rotated . of course , instead of the box wrench 14 , a screwdriver part ( bit ) as shown in fig1 may be attached to the chuck 12 in order to rotate the bearing bolt . the position sensor 20 is provided with a photo detector 21 and a liquid crystal display unit 22 . the photo detector 21 receives a laser beam which is horizontally emitted and rotated at a predetermined rotating speed by a level radiation beam emitting device such as an electronic level device , and the like . the photo detector is provided with a pair of wedge - shaped photo detection elements and generates a pair of corresponding photoelectric signals s1 and s2 , which vary according to the positions of the laser beam traversing the photo detector 21 . such a position sensor 20 is well known , for example , through u . s . pat . no . 4 , 674 , 870 . as shown in fig3 these photoelectric signals s1 and s2 are input to a signal processing circuit 23 contained in the position sensor 20 . the signal processing circuit 23 , having functions such as amplification , holding the peak value , comparison , and so on , processes said photoelectric signals s1 and s2 , generates an output signal a to an output line when the laser beam traverses the lower edge position a of the photo detector 21 , generates an output signal b to the output line when the laser beam traverses the reference position b in the middle of the photo detector 21 , and generates an output signal c to the output line when the laser beam traverses the upper position c of the photo detector 21 . the liquid crystal display unit 22 receives the output signals a , b and c through the output line and displays an arrow pointing downward , a bar and an arrow pointing upward corresponding to said respective output signals . the control box 30 contains a drive control circuit 31 which receives the output signals a , b and c from said signal processing circuit 23 and controls the directions of rotation and rotating speed of the motor 15 provided in the electric driven screwdriver main body 10 . the control box 30 further has red and green display lamps 32 , wherein the red lamp is turned on by turning on a power source switch ( not shown ) and the green lamp is turned on when thread fastening operation is finished by the screwdriver . further , a horizontal posture indicator 40 is fixedly provided on the control box 30 . as shown in fig4 the horizontal posture indicator 40 consists of : a pair of upper and lower dish - shaped transparent plates 41 and 42 made of glass , plastic , or the like ; a steel ball 43 put therebetween ; and a frame 44 for supporting them . each transparent plate 41 , 42 has a spherical surface with its center outwards . on the outer surfaces of the transparent plates 41 and 42 , restriction circles 45 and 46 are drawn , respectively . thus , if the electric driven screwdriver is in the posture as shown in fig1 the steel ball 43 is substantially at the center of the dish - shaped transparent plate 42 , as shown in fig4 . when the screwdriver is turned upside down , the steel ball 43 , then on the inner surface of the dish - shaped transparent plate 41 , is substantially at the center of the plate 41 . in both cases , the electric driven screwdriver can be held substantially in the vertical posture by visually confirming the steel ball 43 being substantially at the center of the restriction circle 45 or 46 . incidentally , instead of the steel ball 43 , the space between the transparent plates may be filled with liquid with a bubble 47 . now , procedure of execution of a floor board with the electric driven screwdriver of this embodiment will be described with reference to fig5 and 6 . on the foundation slab 101 on which many bearing bolts 102 are set up substantially vertically , a level radiation beam emitting device such as an electronic level device , or the like is provided so that a laser beam 111 is horizontally projected and rotated at a predetermined rotating speed . in case of execution of a floor board , the heights of the nuts 104 engaged with corresponding bearing bolts 102 are adjusted so as to position all the nuts 104 in the same horizontal plane . first , the selector switch 13 for switching normal and reverse rotations is switched to the forward rotation mode , one of the nuts 104 is set in the box wrench 14 , and the switch 11 is turned on . during the operation , the steel ball 43 in the horizontal posture indicator 40 should be visually checked so as to be substantially at the center of the restriction circle . thus , the electric driven screwdriver is held substantially in the vertical posture during operation . in the above state , as the laser beam 111 is projected below the lower edge of the photo detector 21 , the photo detector 21 does not generate any photoelectric signals . accordingly , the motor rotates at a high speed of ca . 600 rpm . as a result , the nut 104 descends according to the pitch of the thread ( step s101 in fig6 ). as the electric screwdriver descends together with the nut , the lower edge of the photo detector 21 enters the track of rotation of the laser beam 111 , when the photoelectric signals s1 and s2 ( s1 & gt ; s2 ) are output from the photo detector 21 . the signal processing circuit 23 judges , on the basis of these photoelectric signals , that the beam is received at a position between a and b of the photo detector , shown in fig3 and generates the output signal a to the output line . the liquid crystal display unit 22 receives the output signal a and displays the arrow pointing downwards . and the drive control circuit , when the output signal a is received , decelerates the rotating speed of the motor to one third thereof ( steps s102 and s103 ). thus , the nut 104 descends slowly . when the middle of the photo detector 21 coincides with the track of rotation of the laser beam 111 , said pair of photoelectric signals satisfies the equation s1 = s2 . then , the signal processing circuit 23 judges that the beam is received at the position b shown in fig3 and generates the output signal b to the output line . the liquid crystal display unit 22 receives this output signal b and displays the bar , and the drive control circuit 31 stops the motor ( steps s104 and s105 ). at this time , the nut 104 has been automatically positioned at a height directed by the laser beam 111 , that is , a series of thread fastening operations is finished . if the motor can not be stopped at the moment the output signal b is generated in the output line for some reason such as torque , the track of rotation of the laser beam goes beyond the position b . in this case , said pair of photoelectric signals has a relation s1 & lt ; s2 . thus , the signal processing circuit 23 judges that the beam is received at a position between b and c of the photo detector , shown in fig3 and generates the output signal c in the output line . the liquid crystal display unit 22 , when the output signal c is received , displays the arrow pointing upwards . the drive control circuit 31 rotates the motor reversely at low speed ( steps s106 and s103 ), and stops the motor when the output signal b is generated again . in the similar way , all the nuts are positioned with respect to corresponding bearing bolts . and the floor board 103 is put on these nuts as indicated the imaginary lines in fig5 . thus , the floor board 103 can be set horizontally at a predetermined height directed by the laser beam 111 . fig7 shows the operation of horizontal execution of a ceiling board . in this execution , many nuts 204 are engaged with corresponding bearing bolt 202 set up substantially vertical on a ceiling foundation slab 201 . accordingly , the electric driven screwdriver is used upside down , wherein the steel ball 43 in the horizontal posture indicator 40 rolls over the inner surface of the other dish - shaped transparent plate 41 . therefore , also in this case , the operator can work while visually checking the steel ball 43 so that the steel ball is substantially at the center of the restriction circle . the procedure of execution is the same as that of execution of the floor board . though , in the first embodiment , the rotating speed of the motor is controlled by two stages , the rotating speed may be decelerated by multistages or continuously as the laser beam approaches the reference position , wherein the distance between the laser beam and the reference position , that is , the target , is judged by comparing said pair of photoelectric signals s1 and s2 . in the above - mentioned first embodiment , the position sensor 20 is fixed to the electric driven screwdriver 10 , and the photo detector is also shifted vertically together with the screwdriver . and in the above - mentioned execution , the nuts 104 engaged with the bearing bolts 102 are rotated so that all the nuts 104 are positioned in the same horizontal plane . but , in the execution of the floor board as shown in fig1 , in which the bearing bolts set up on the foundation slab are rotated to shift the floor board vertically together with the nuts , the electric driven screwdriver itself does not shift vertically even if the bearing bolts are rotated . therefore , if the beam - emitting device 110 is installed on the foundation slab 101 as shown in fig5 relative displacement between the laser beam 111 and the electric driven screwdriver can not be obtained . even when the beam - emitting device itself is installed on the floor board which is shifted vertically , the execution of the floor board is practically impossible for the reason explained below . as shown in fig8 revolvable bearing bolts 302 are provided on a foundation slab 301 and nuts 304 engaged with respective bearing bolts 302 are fixedly provided in a floor board 303 . a position sensor 320 is fixed to an electric driven screwdriver main body 310 . and a beam emitting device 410 set on a tripod is installed on the floor board 303 . a level radiation beam emitting device 410 such as an electronic level device is generally provided with an automatic level correction mechanism so that a laser beam 411 is always projected horizontally regardless of inclination of the housing . accordingly , even when the floor board 303 is slanted as indicated by the solid lines in fig8 the laser beam 411 emitted from the beam emitting device 410 travels horizontally . in order to horizontalize the floor board 303 , the electric driven screwdriver positioned as shown in fig8 is driven to rotate the bearing bolt 302 . but , even when the floor board 303 is horizontalized as shown by the imaginary lines , the positional change of the laser beam 411 in the vertical direction with respect to the position sensor 320 is very little . therefore , the photoelectric signals corresponding to the shift of the floor board can not be obtained from the position sensor 320 fixedly provided with respect to the electric driven screwdriver main body 310 . so , in the second embodiment according to the present invention , a position source provided to the electric driven screwdriver main body can be slided with respect to the main body so as to be shifted vertically together with the floor board . this second embodiment will be described below in detail with reference to fig9 to 12 . in fig9 and 10 , an electric driven screwdriver main body 510 is provided with a bit 514 fixed to a chuck at the end of a drive shaft and a supporting frame 515 fixed on a side of a housing . as is shown in detail in fig1 , the supporting frame 515 has a guide shaft 516 whose ends are held by a pair of flanges 517 and 518 . a photo sensor 521 is provided on the front surface of the position sensor 520 , while a slider 522 engaged with the guide shaft 516 is fixed to the rear surface of the position sensor 520 . by means of the slider 522 , the position sensor 520 and the electric driven screwdriver 510 can be shifted from each other in the direction of the drive shaft . by operating a clamp knob 523 , the slider 522 can be fixed to the guide shaft 516 . a horizontal posture indicator 540 which is the same as that in the above - mentioned first embodiment is fixedly provided on the upper part of the side surface of the position sensor 520 . also a supporting plate 550 is fixed to the lower part of the opposite side surface of the position sensor 520 . the supporting plate 550 consists of a pair of pieces 551 and 552 fastened together with wing nuts 553 , wherein the length of the supporting plate 550 is adjustable . the upper end portion of one piece 551 is fixed to the housing of the position sensor 520 , while the lower end portion of the other piece 552 is bent orthogonally to form a footboard 554 . procedure of execution of a floor board according to the second embodiment will be described with reference to fig1 . first , the length of the supporting plate 550 is adjusted so that the length between the detection center of the photo detector 521 and the footboard 554 at the lower end portion of the supporting plate 550 becomes equal to ( h1 - h2 ), wherein h1 is the height of a laser beam 611 projected from a beam emitting device 610 from a foundation slab 501 , and h2 is the planned heights of the upper surface of the floor board 503 from the foundation slab 501 . next , the bit 514 of the electric driven screwdriver is engaged with the bearing bolt 502 . and the operator treads on the footboard 554 with his or her foot to set up the supporting plate 550 and the position sensor 520 vertically . then , verticality of the posture of the apparatus is confirmed by means of the horizontal posture indicator 540 . when a power source switch ( not shown ) is turned on to rotate the bearing bolt 502 , the floor board 503 together with the nut 504 descends . in this case , though the electric driven screwdriver 510 stands still , the slider 522 slides along the guide shaft 516 as the floor board descends . thus , the supporting plate 550 and the position sensor 520 descend . when the center of the photo detector 521 gradually approaches a laser beam 611 and finally coincides with the track of rotation of the laser beam 611 , a pair of photoelectric signals satisfies the equation s1 = s2 as in the first embodiment and the drive control circuit stops the motor . at this time , the floor board 503 is automatically positioned at the height directed by the laser beam 611 , that is , a series of thread fastening operations is finished . and the floor board 503 is horizontalized at the predetermined height as indicated by the imaginary lines in the drawing . incidentally , in case of executing a ceiling horizontally according to the second embodiment , the supporting plate 550 is not used . in this case , the center of the photo detector 521 is positioned by fixing the slider 522 with the cramp knob 523 . in addition , in the second embodiment , a scale 555 indicating the above - mentioned length h1 - h2 may be cut in the piece 552 of the supporting plate for convenience .