A circuit for integrating a quantity of light in an automatic control type flash unit having flash means for providing flash illumination comprises light responsive means, an amplifier circuit, a high-pass filter, a voltage-current conversion circuit and a capacitor. The light responsive means receives light resulting from the flash illumination and converts the received light into a corresponding electrical signal. The amplifier circuit puts out the electrical signal as an amplified voltage. This circuit also puts out DC and low frequency error voltages. The high-pass filter eliminates DC and low frequency error voltages from the voltage put out by the amplifier circuit and puts out only the voltage corresponding to the electrical signal. The conversion circuit puts out a current corresponding to the voltage put out by the high-pass filter. The capacitor is charged with the current from the voltage-current conversion circuit. The charging voltage of the capacitor assumes a value which is the result of integration of the intensity of the light received by the light responsive means.