PURPOSE: To ensure the accurate measurement for shutter exposure time without receiving the effect of a drift, etc., by calculating the exposure time through the process of a computer in place of a comparator.

CONSTITUTION: The shutter opening/closing waveform obtained by a photodetector PC is amplified up to a proper level through an amplifier AMP and then fed to an A/D converter ADC. This input signal is sampled synchronously with the clock pulse supplied from an oscillator OSC and then successively converted into the digital code to be supplied to a computer CP which functions as a storage arithmetic means. The computer CP stores the sample value corresponding to the sampling time and sets the reference value from the storage value. Then two sampling times corresponding to the two sample values equal to the reference value are obtained to calculate the exposure time. In such a way, the shutter exposure time can be measured with high accuracy and without receiving an effect of a drift, etc.