52 in a time series in synchronization with a set read scan cycle and sequentially converted by an a d converter 53 into pixel data d for each set bit these pixel data d are supplied to a signal analyzing circuit 55 with a computing function as of a microcomputer or others through an interface circuit 54 in fig2 the portions indicated by the mark cn are optical connecting means such as optical connectors and others then the operation of this embodiment will be explained when an operator inserts an optical fiber coupler 48 to be measured between the input mechanism and the output mechanism and the measurement is started light of set wavelengths from the light sources 42 45 are mixed by the optical fiber coupler 46 and the mixed light is incident on the input optical fiber 47a of the optical switch 47 the optical switch 47 outputs in response to a switch signal ch1 the light incident thereon to one 47b of the output optical fibers of the optical switch 47 the output light of the output optical fiber 47 of the optical switch 47 is incident on the input optical fiber 48a of the optical fiber coupler 48 to be measured coupled thereto by the optical connector the mixed light is separated in accordance with the respective spectral characteristics of the output optical fibers 48c 48f the light corresponding to a spectral characteristic of the input optical fiber 48a to the output optical fiber 48c is supplied to the first input.