PURPOSE: To accurately measure the dimension of optical connector by keeping an end face space in the butt direction against a light sending/receiving ferrule to be a space as desired on the basis of the correlation between the end face space and transmission loss.

CONSTITUTION: While a Z-axis stage is being shifted vertically, the transmission loss is measured at respective shifting positions to prepare a correlation diagram showing the transmission loss against the end face space. On the other hand, the intensity of light transmitted from a light sending/receiving fiber 8 is measured by an optical power meter. Then a computer calculates an end face interval Z between a multi-core connector 2 and a light sending/receiving ferrule 7 based on the correlation diagram with the reference to the optical signal inputted from the optical power meter, and outputs a correction signal to a Z controller based on the interval Z to correct the stage 12. Furthermore, after the ferrule 7 is changed over, the positions of respective optical fibers 3a and 6 are found out with the computer. Thus a plurality of positions of fiber 3a of the connector 2 can be obtained based on that a pin hole 2c is a reference hole when either is set to the reference.