fig1 a , 1 b and 2 show a structure of an electronic endoscope apparatus which is an observed image forming apparatus in an embodiment according to the invention . fig1 a and 1b show a structure related to insertion and extraction of a cap into and from a connector receiver of a processor unit , and fig2 shows an overall structure of the electronic endoscope apparatus . in the beginning , as shown in fig2 , in the electronic endoscope apparatus , a connector 14 a of a scope 10 a of type a ( an observation device — an electronic endoscope ) is connected to a connector receiver 14 b of a processor unit ( an apparatus body ) 12 , and this processor unit 12 is provided with a connector receiver 15 b for connecting another scope of type b . the scope 10 a contains therein a ccd 17 which is a solid body photographing element , a cds ( correlation double sampling )/ agc ( auto gain control ) circuit 18 for inputting an image taking signal which has been obtained by this ccd 17 , and so on . on the other hand , the processor unit 12 contains therein a ccd driving pulse circuit 20 for driving the ccd 17 , a scope power supplying circuit 21 for supplying necessary power to the scope 10 a , an a / d converter 22 for inputting an output of the cds / agc circuit 18 , an image processing circuit 23 which performs a determined image processing of the output signal of this a / d converter 22 , and a cpu ( or a microcomputer ) 24 which totally controls these circuits and also controls insertion and extraction of the cap . in this embodiment , there is provided a cap ( in a round shape , for example ) 15 c which is mounted and connected to the aforesaid connector receiver 15 h ( as for the other connector receiver 14 b , a cap in a square shape for example is provided ), and at the same time , a cap insertion and extraction detecting circuit 26 is connected to the connector receivers 14 b , 15 b . a structure related to the cap 15 c , the connector receiver 15 h and the cap insertion and extraction detecting circuit 26 is shown in fig1 a and 1b . a photo - interrupter ( an insertion detecting circuit ) 28 for detecting an inserted state of the cap 15 c is provided on a side face of the connector receiver 15 b . this photo - interrupter 28 has a light emitting part ( such as a light emitting diode ) and a light receiving part ( a photo - transistor ), and can detect the inserted state of the cap 15 c with respect to the connector receiver 15 b , by reflecting a light emitted from the light emitting part from the side face of the cap 15 c , and by detecting the reflected light in the light receiving part . a plurality of pins ( electrodes ) 30 are arranged in the connector receiver 15 b , while two pin receivers ( electrodes ) 31 are provided in the cap 15 c ( arrangement of these pins and pin receivers may be reversed or some other structures may be employed ). these pin receivers 31 make short circuits inside . moreover , the connector receiver 15 b is provided with a pin connection detecting circuit 33 for detecting connection between the pins 30 and the pin receivers 31 , and an and circuit 34 is provided on an after stage of the pin connection detecting circuit 33 and the aforesaid photo - interrupter 28 . specifically , in the cap insertion and extraction detecting circuit 26 in this embodiment , when the cap 15 c is mounted to the connector receiver 15 b , as shown in fig1 b from the state as shown in fig1 a , the insertion of the cap 15 c itself is detected by the photo - interrupter 28 , and the connection between the pins 30 and the pin receivers 31 is also detected by the pin connection detecting circuit 33 . at this moment only , a signal ( high signal ) for judging that the cap 15 c has been mounted ( mounting of the cap ) is issued . this embodiment has the above described structure , and the electronic endoscope apparatus in this embodiment is so constructed that a plurality of the scopes 10 a or the like can be connected to the processor unit 12 , as described referring to fig5 a and 5b . for example , in case where the connector 14 a of the scope 10 a is connected to the connector receiver 14 b of the processor unit 12 , as shown in fig2 , the cap 15 c is mounted to the other connector receiver 15 b . on this occasion , operation as shown in fig4 is performed in this embodiment . in fig4 , when the mounting of the cap 15 c has been performed as described above , it is judged in step 101 whether or not the cap insertion and extraction detecting circuit 26 has detected the insertion of the cap 15 c . then , as described referring to fig1 b , when the insertion of the cap 15 c has been detected by the photo - interrupter 28 , and the connection between the pins ( 30 , 31 ) has been detected by the pin connection detecting circuit 33 , it is judged that the cap 15 c has been mounted ( yes ). then , in step 102 , the cpu 24 energizes the ccd driving pulse circuit 20 and the scope power supplying circuit 21 , whereby supply of ccd driving pulse and supply of scope power to the ccd 17 of the scope 10 a is started ( step 103 ). then , in step 104 , the cpu 24 requests to display an image observed by the scope , and a determined signal processing is performed on an image signal which has been inputted from the scope 10 a , in the image processing circuit 23 , and the observed image is displayed on a monitor ( step 105 ). on the other hand , in case where the cap 15 c is not yet mounted , has been detached or has been extracted , as shown in fig1 a , the insertion of the cap 15 c is not detected by the photo - interrupter 28 ( non - insertion is detected ), the connection of the pins is not detected by the pin connection detecting circuit 33 ( non - connection is detected ) or both are not detected in the aforesaid step 101 , and it is judged that the cap 15 c is not mounted ( no ). in this case , the cpu 24 stops the ccd driving pulse circuit 20 and the scope power supplying circuit 21 in step 106 , and therefore , supply of the ccd driving pulse to the ccd 17 and supply of the scope power is stopped ( 107 ). then , in step 108 , the cpu 24 requests a message that the cap 15 c should not be extracted , and the massage to the effect that the cap 15 c should not be extracted is displayed on the monitor or the like ( step 109 ). in the above described cap insertion and extraction detecting circuit 26 , only when the insertion of the cap 15 c itself has been detected by the photo - interrupter 28 and the connection of the pin receivers 31 of the cap 15 c to the pins 30 of the connector receiver 15 b has been detected by the pin connection detecting circuit 33 , the scope 10 a is driven to operate . therefore , through this double detection , forgetting to mount the cap 15 c and incompletely mounting can be reliably prevented . fig3 shows another example of the structure of the insertion detecting circuit in the cap insertion and extraction detecting circuit 26 . in the insertion detecting circuit in this example , there are provided an operating rod 36 which is supported so as to reciprocally move to the right and left in the drawing , a micro switch 37 arranged at a back end side of this operating rod 36 , and a switch on / off detecting circuit 38 for detecting of / off of the micro switch 37 . with the insertion detecting circuit in this structure , when the operating rod 36 is pushed by the insertion of the cap 15 c , the micro switch 37 is turned on , whereby the insertion of the cap 15 c is detected . therefore , in the cap insertion and extraction detecting circuit 26 , only when turning on of the micro switch 37 has been detected by this switch on / off detecting circuit 38 , and the connection of the pins has been detected by the pin connection detecting circuit 33 , an output of the and circuit 34 becomes high . as the results , it is judged that the cap 15 c has been mounted . although in the above described embodiment , it is so adapted that operation of the scope 10 a starts , when the mounting of the cap 15 c has been detected by the cap insertion and extraction detecting circuit 26 , it is also possible to start operation of the processor unit 12 including the scope 10 a , that is , operation of the electronic endoscope apparatus . moreover , although the invention is applied to the electronic endoscope apparatus in the embodiment , it is also possible to apply the invention to a complex observation apparatus in which a scope is connected to an ultrasonic probe , for example . according to the observed image forming apparatus of the invention , there is obtained such advantage that electrical safety of the internal circuit can be secured , even in case where the cap is forgotten to be mounted , or incompletely mounted , and such possibility that a user may get in touch with the electrode inside the connector receiver can be eliminated . moreover , according to the second aspect of the invention , forgetting to mount the cap and incomplete mounting of the cap can be reliably prevented through the double detection . the entire disclosure of each and every foreign patent application from which the benefit of foreign priority has been claimed in the present application is incorporated herein by reference , as if fully set forth .