A PV vector measuring apparatus includes: a light source 101configured to output probe light; a polarization switch 102 that can freely set a state of polarization of input light; a polarimeter 104; a PV-vector calculating device 105; and a rectangular wave generator 106. The polarization switch alternately switches between two orthogonal states of polarization in accordance with a rectangular wave modulation signal output from the rectangular wave generator. Output light from the polarization switch is input to a measurement object 103. The polarimeter 104 measures time dependency of an SOP vector of output light output from the measurement object. The PV-vector calculating device calculates a characteristic vector, which expresses a rate of polarization change in the measurement object, from the time dependency of the SOP vector.