A camera generates a light source control signal for controlling a light source to turn the light source on and off, acquires a plurality of items of first image data and a plurality of items of second image data obtained by picking up the measured wave, and transfers the plurality of items of acquired first image data and the plurality of items of acquired second image data to an external storage and processing device as digital data with an identifying number for identifying a frame in which the image data is acquired attached every frame. The external storage and processing device determines whether each frame of the digital data is an item of the first image data or an item of the second image data on the basis of the identifying number, and subtracts the plurality of items of determined second image data from the plurality of items of determined first image data to produce difference image data.