an embodiment of a shock sensor according to the present invention is shown in fig2 and fig3 . a shock sensor 11 comprises a box like case 12 and a sensor unit 13 . the sensor unit 13 is accommodated in an accommodating portion 12a of the case 12 . a lid 14 having a transparent window 14a ( shown by a dotted chain line in fig2 ) is fixed on a front face 12b of the case 12 . as best illustrated in fig4 a fastening hole 12c is provided on each side of the case 12 , the shock sensor 11 being fixed to an object by using these holes 12c by means of screw . the accommodating portion 12a is formed in the case 12 so that the case 12 has a space having a nearly rectangular shaped opening . a fixing portion 12e having grooves 12d provided for fixing the sensor unit 13 protrudes inside of the accommodating portion 12a . as shown in fig5 the sensor unit comprises a u - shaped fixing member 15 and a pair of supporting members 16 , 17 ( a first and a second supporting members ), which are formed of thin metal plate and supported by the fixing member 15 . a weight 18 is fixed to an end 16a of the first supporting member 16 , and an indicating plate 19 is formed on a side of an end 17a of the second supporting member 17 . fixing member 15 is comprised of three fixing plates 15a , 15b , and 15c . fixing plates 15b and 15c are substantially parallel plates corresponding to the two upwardly extending less of the &# 34 ; u &# 34 ;. fixing plates 15b and 15c rest on bottom fixing plate 15a , the latter corresponding to the cross - member of the &# 34 ; u &# 34 ;. the fixing plates 15b and 15c have v - shaped grooves 15d and 15e on their respective side faces so as to support the supporting members 16 and 17 . the supporting member 16 and 17 are formed in nearly the same shape and are respectively provided with openings 16b and 17b into which the fixing plates 15b and 15c of the fixing member 15 are respectively inserted . the upper supporting member 16 is placed on an upper surface of the lower supporting member 17 . inner edges 16c and 17c of the openings 16b and 17b of the two supporting members 16 and 17 are joined together and engaged with the v - shaped groove 15d formed on a side face of the fixing plate 15b . accordingly , the supporting members 16 and 17 are rotatably supported by the inner edges 16c and 17c so that the opposite sides of the supporting members 16 and 17 where the weight 18 is fixed can move up and down . it is should be noted that the supporting member 16 provided with the weight 18 and the supporting member 17 provided with the indicating plate 19 are separate members which can move independently from each other . the lower supporting member 17 is provided with a spring portion 17d ( a holding portion ) which extends from an inner edge of the opening 17b , and an edge of the spring portion 17d is engaged with the v - shaped groove 15e of the fixing plate 15c . the sensor unit 13 of the above mentioned construction is , as indicated by an arrow in fig5 inserted into the accommodating portion 12a of the case 12 . when inserting the sensor unit 13 into the case 12 , the respective ends of the fixing plates 15b and 15c are inserted into the grooves 12d and thus the sensor unit 13 is fixed at a predetermined position inside the case 12 . in a state where the shock sensor 11 is assembled , as shown in fig5 and fig8 a , the ends 16a and 17a of the supporting members 16 and 17 are maintained to be in positions higher than the v - shaped groove 15e . this is because a distance between an edge of the spring portion 17d and the inner edge of the supporting member 17 which is engaged with the v shaped groove 15d is slightly shorter than a distance between the two v shaped grooves 15d and 15e . such a relationship between these distances results in the spring portion 17d being bent when it is assembled . that is , when assembling the sensor 11 , the sensor unit 13 is assembled so that the spring portion 17d is bent upward into an arc - like shape , with the result that the ends 16a and 17a of the supporting members 16 and 17 located at positions higher than the groove 15e . an operation of the sensor unit 13 will be described below . before an acceleration is applied to the sensor unit 13 , as shown in fig5 and fig8 a , the supporting members 16 and 17 are maintained in positions in which their ends 16a and 17a are higher than the v - shaped groove 15e . as shown in fig6 and fig8 b , when a downward acceleration g is applied to the weight 18 , a downward pressing force is applied to the end 16a of the upper supporting member 16 , and the same pressing force applied to the supporting member 16 is then in turn applied to the end 17a of the lower supporting member 17 . accordingly , the ends 16a and 17a of the supporting members 16 and 17 move downward . when the pressing force exceeds a predetermined value and the ends 16a and 17a being to move a large distance , a buckling occurs in the spring portion 17d , which results in deformation of the spring portion 17d , from an arc - like shape bent upward to an arc - like shape bent downward . in this state , the ends 16a and 17a are in positions lower than the v - shaped groove 15e and the supporting members 16 and 17 are maintained in these positions by a downward elastic pressing force generated by the spring portion 17d . at this time , the indicating plate 19 provided on the side of the end 17a of the supporting member 17 also moves to a position lower than the v shaped groove 15e , that is a shock detected position . the transparent window 14a of the lid 14 is provided in a position where the indicating plate 19 can be seen through the window 14a when the indicating plate 19 is at the lower shock detected position . since the indicating plate 19 is colored with such bright colors as red or yellow , it is easy to recognize the indicating plate 19 when it is opposite to the window 14a , and accordingly , it can be instantly recognized that an acceleration greater than a predetermined value has been applied to the sensor 11 due to external shock or vibration . in this embodiment , since the weight 18 is fixed to the supporting member 16 , the supporting member 16 is directly pressed without any friction resistance between the weight 18 and the case 12 , and the supporting member 17 is pressed at the same time , thus assuring that a reliable operation for detecting an acceleration can be performed . as shown in fig7 and fig8 c , in a case where a reverse ( upward ) acceleration g &# 39 ; is applied after the acceleration detecting operation has been performed , the upper supporting member 16 moves upward while the lower supporting member 17 is maintained at the position lower than the v - shaped groove 15e by means of the spring portion 17d of the supporting member 17 . accordingly , the indicating plate 19 does not move from the lower shock detected position opposite to the window 14a , which position indicates an acceleration has been detected . as a result , even if a reverse acceleration g &# 39 ; is applied to the sensor 11 after the detection of the acceleration , the indicating plate 19 will not move away from the window 14a . therefore , errors , such as an operator assumes the sensor has not detected an acceleration because the indicating plate 19 has moved away from the window 14a after an acceleration has been detected , are eliminated . as shown in fig8 d , after the reverse acceleration g &# 39 ; is suppressed , the supporting member 16 having the weight 18 fixed thereon returns to the lower position . as mentioned above , the weight 18 and the indicating plate 19 are respectively supported by the supporting members 16 and 17 so that the weight 18 and the indicating plate 19 move downward together when an acceleration g is applied and the weight 18 independently moves upward when a reverse acceleration g &# 39 ; is applied . as in the sensor of the present invention there is no friction resistance due to the sensor being provided with the above mentioned supporting members 16 and 17 , the weight 18 is lighter and smaller than the weight of the above mentioned conventional sensor , and thus the sensor of the present invention is simpler and more compact than the conventional sensor . the present invention is not limited to the specifically disclosed embodiment , and variations and modifications may be made without departing from the scope of the present invention .