An electron multiplication gain calibration mechanism, which is capable of readily stabilizing a gain of an electron multiplying CCD image pickup device, is provided. The mechanism comprises a gain adjusting section and a control section. The gain adjusting section is provided independently of gain adjustment by the electron multiplying CCD image pickup device, and sets a reference gain. The control section calibrates the gate voltage for obtaining a desired electron multiplication gain by comparing a video level amplified by the electron multiplication gain to a video level amplified by the reference gain. The control section sets an initial value of the gate voltage, which is supposed to provide the electron multiplication gain equal to the reference gain, and controls the gate voltage gradually so that the video level amplified by the electron multiplication gain becomes equal to the video level amplified by reference gain.