some embodiments of the present invention will hereinafter be described with reference to the drawings . as shown in fig1 and 2 , a u - shaped positive pole contact piece 14 and a plate - like negative pole contact piece 16 are secured to the surface 12 of a camera body 10 , and a limit screw 18 is mounted on said surface . a recess 20 for containing therein main power source batteries 50a and 50b as a first power source battery is formed in one end portion of said surface 12 . a disk - like auxiliary power source battery 60 as a second power source battery is placed on the surface 12 , and the positive pole contact piece 14 and the negative pole contact piece 16 are in contact with the positive pole and the negative pole , respectively , of the battery 60 . a bottom cover 24 is disposed outside the battery 60 and attached to the camera body 10 . the bottom cover 24 presses the battery 60 by its thick portion 26 and is provided with a cut - away 28 in the fore end portion thereof . a battery lid 30 is disposed outside the batteries 50a and 50b , and this lid 30 is provided with a bearing portion 36 which bears against an l - shaped engaging portion 32 resiliently deformable and engaged with the cut - away 28 of the bottom cover 24 and against the limit screw 18 , and is biased rightwardly by the resiliency of the engaging portion 32 . a contact piece 36 for connecting the negative pole of the battery 50a and the positive pole of the battery 50a is secured to the inner surface of the lid 30 . one end portion 41 of this contact piece 36 is engageable with the curved portion 43 of the camera body 10 . an electric system supplied with an electric power by the battery 50 ( the batteries 50a and 50b together constitute a battery 50 ) and the battery 60 will now be described with reference to fig7 . cpu 100 is comprised of an ic and is operated by the auxiliary power source 60 . a display unit 101 displays the photographing information of the camera ( such as film speed and the number of photographed film frames ) by a driver 111 in accordance with the signal of cpu 100 . as regards an electronic flash device 102 , the emission of flashlight to an object to be photographed is controlled by cpu 100 . a shutter 103 has shutter blades for lens shutter , a magnet for opening the shutter blades and a magnet for closing the shutter blades , and is controlled by cpu 100 through a driver 110 . a motor 104 effects the winding - up of the film from a film cartridge loaded into the camera and the rewinding of the film into the cartridge , by a signal supplied from cpu 100 through a driver 109 . a motor 105 drives a photo - taking lens 114 . cpu 100 controls a driver 108 and drives the motor 105 in accordance with a signal from a range finding circuit 107 indicative of the distance to the object to be photographed so that the image of the object to be photographed by the photo - taking lens 114 is focused to a predetermined plane of the camera . a photometering circuit 106 inputs to cpu 100 a signal indicative of the brightness of the object to be photographed , and a counter 112 has a switch adapted to be mechanically closed each time the film is wound up from the cartridge by a frame for photography , and cpu 100 counts the pulses generated by on - off of this switch . a film information detecting device 113 is constituted by a plurality of electrodes which are in contact with a pattern comprising an insulating portion and an electrically conductive portion for indicating the film speed which are formed on the surface of the film cartridge . cpu 100 reads the film speed by the plurality of electrodes of the film information detecting device 113 , reads the output of the photometering circuit 106 and causes the driver 110 to control the two magnets of the shutter 103 . the main power source battery 50 spplies an electric power to the display unit 101 , the electronic flash device 102 , the shutter 103 , the motors 104 and 105 , the photometering circuit 106 , the range finding circuit 107 and the drivers 108 - 111 , and the auxiliary power source battery 60 supplies an electric power to cpu 100 . cpu 100 also has an inner memory for storing the read film speed and the number of photographed frames therein . the number of lines from cpu 100 to other circuit or device is one , and it is to be understood that this includes a parallel digital signal comprising a plurality of lines . this also holds true of the line from the device 113 to cpu 100 . operation of the present embodiment will now be described . to interchange the auxiliary power source battery 60 , the engaging portion 32 is first resiliently deformed and the battery lid 30 is moved leftwardly as viewed in fig2 . thereupon , the engagement between the end edge portion 41 of the contact piece 36 and the curved portion 43 of the camera body is released and therefore , the lid portion 37 of the battery lid 30 is opened upwardly about the bent portion 35 as indicated by dots - and - dash line in fig2 . at this moment , the contact piece 36 separates from the negative pole of the battery 50a and the positive pole of the battery 50 and the power supply from the battery 50 is stopped . when the lid 30 is opened , the battery 50 can be taken out . subsequently , when the limit screw 18 is removed from the camera body 10 ( this is accomplished by pushing the lid 30 leftwardly and resiliently deforming the engaging portion 32 ), the restraint of the foot portion 34 by the screw is released and therefore , if the battery lid 30 is drawn out rightwardly as viewed in fig4 the auxiliary power source battery 60 will be pushed out rightwardly by the resilient force of the positive pole contact piece 14 , as shown in fig3 and 4 . to interchange the main power source battery 50 , only the first half of the above - described procedure may be effected . after the consumed battery 60 has been taken out in accordance with the above - described procedure , a new auxiliary power source battery 60 is inserted in accordance with a procedure converse to the above - described procedure . that is , the new battery 60 may be contained into the void between the camera body 10 and the cover 30 , the battery lid 30 may be positioned at a predetermined position and restrained by the limit screw 18 , the lid portion 37 may be opened and the main power source battery 50 may be contained into the void 20 , whereafter the lid 30 may be moved leftwardly and the endedge portion 41 may be brought into engagement with the curved portion 43 , and then the lid 30 may be closed . as described above , there is only one battery lid 30 in the present embodiment , and when the main power source battery 50 has been consumed , only this battery is taken out , and when the auxiliary power source battery 60 has been consumed , the main power source battery 50 and the auxiliary power source battery 60 are taken out ( the interchange of the batteries 50 and 60 is effected with the lid 30 opened ). that is , it is not possible that only the auxiliary power source battery 60 is taken out and in the meantime , the power is supplied from the main power source battery 50 . accordingly , it never happens that the motor 104 , etc . are caused to malfunction or the ic is destroyed by the power supply only from the battery 50 . also , when the battery 50 is to be taken out , the lid portion 37 of the lid 30 may be opened upwardly , and when the battery 60 is to be taken out , the entire lid 30 may be drawn out rightwardly and therefore , malfunctioning is eliminated and the space occupied is small . another embodiment of the present invention will now be described with reference to fig5 and 6 . for simplicity of description , portions of this embodiment similar to those of the previous embodiment are given similar reference numerals and need not be described , and only different portions will hereinafter be described . a feature of the present embodiment is that a battery lid 130 is provided with an extension 132 extending toward the auxiliary power source battery 60 and this extension is formed with a cut - away 134 for containing and holding the battery 60 therein . when the battery 60 is to be interchanged , the battery 50 is taken out , whereafter the screw 18 is taken out , and if the lid 130 is moved rightwardly ( as viewed in fig6 ) and drawn out , the battery 60 can be taken out while being held by the lid 130 and thus , the trouble of taking out the battery 60 is saved . in the present embodiment , the biasing force for bringing the end edge portion 41 of the contact piece 36 fixed to the lid 130 into engagement with the curved portion 43 of the camera body 10 is provided by a spring 14 . the above - described embodiments are merely a part of the present invention and of course , the present invention is never restricted to these embodiments . for example , in the previously described embodiments , the battery lid of the battery 50 and the battery lid of the battery 60 are provided by the single lid 30 or 130 , whereas this is not indispensable , but the battery lids of the batteries 50 and 60 may be provided discretely and design may be made such that the power supply circuit of the main power source battery 50 is cut off by a switch adapted to be opened in response to a preliminary operation for cutting off the power supply by the auxiliary power source battery 60 , for example , the operation of releasing a lock device for locking the battery lid of the auxiliary power source battery 60 . also , in the second embodiment shown in fig5 and 6 , when the auxiliary power source battery 60 is to be taken out , the battery lid 130 need not always separate from the bottom cover 24 , but may be adapted to slide on this cover . according to the present invention , as has hitherto been described , it never happens that the power is supplied only from the first power source battery and therefore , destruction of the ic or malfunctioning of the display and counter is prevented . this also means that any electric circuit for protecting the ic from said malfunctioning becomes unnecessary , and the number of parts is reduced , whereby the cost of manufacture is reduced and the space required for the power supply device can be made small .