Wavemeter

1. Field of the Invention A wavemeter for measuring a signal light which is obtained by multiplexing a plurality of optical signals in a predetermined system and which is transmitted, comprises: a reference setting unit for setting a reference value for measurement of the signal light obtained by the predetermined system, a measuring unit for continuously measuring the transmitted signal light, a specifying unit for specifying each optical signal multiplexed into the signal light, in accordance with a result of a measurement carried out by the measuring unit, a variation measuring unit for calculating a variation in a physical value of each optical signal specified by the specifying unit in accordance with the reference value for measurement, and an output unit for outputting a transition of the variation calculated by the variation measuring unit.