PURPOSE: To improve measurement accuracy by providing a calibration signal generation part at an optical fiber type control transformer(CT).

CONSTITUTION: When a test mode is selected by pressing a calibration button 21, a calibration signal, for example a light signal corresponding to AC5A is generated by a calibration signal generation part inside CT1A for detection and is inputted to an output part body 2A via an optical fiber cable 4. The inputted light signal (AC5A) is converted to an electrical signal by a photoelectric converter incorporated in the body 2A and AC5A is obtained from an AC output terminal 12 or a signal equivalent to for example DC100mV (equivalent to AC5A) is obtained from a DC output terminal 13, but the light signal is attenuated due to the error peculiar to light transmission. For compensating the amount of attenuation, a compensation dial 22 is rotated to determine the amount of AC amplification. For example, assuming that the test mode is selected and 97mV is outputted from the DC output terminal 13, the dial 22 is rotated while monitoring a voltmeter connected to the DC output terminal 13 to adjust the amount of amplification until the value reaches 100mV.