PURPOSE: To accurately perform automatic focus matching even if a light source output varies by setting the variation rate of the output of one photodetecting element so that a differential amplification output is '0' in an in-focus state when photoelectric outputs of two photodetecting elements are amplified differentially.

CONSTITUTION: A voltage division ratio (s) is set in the in-focus state by varying the resistance value of the variable resistance VR of a voltage dividing circuit DV so that the output e0 of a differential amplifier DA is '0'. Here, when the resistance value of the resistance VR is R2, the voltage division ratio (s) is R2/(R2+R1), where R1 is the resistance value of the resistance R. Therefore, the output voltage e1 of a photodetecting element P1 for focus detection is inputted to one input terminal of the amplifier DA with said characteristics and the output voltage e2 of a photodetecting element P2 for focus detection is divided by the voltage division ratio (s) and inputted to the other input terminal. Then the output is invariably '0' on condition that e1/e2=R2/(R1+R2), thereby judging that the focus is matched. Therefore, a focus matching position is detected without being affected by an offset as long as the ratio of the voltages e1 and e2 is held in said relation.