1. A polarization velocity vector measuring apparatus comprising: a light source configured to output probe light; a polarization switch configured to alternately switch between two orthogonal states of polarization of the probe light and output measurement probe light; a polarimeter configured to measure time dependency of a state-of-polarization vector of output light output from a measurement object that has received the measurement probe light; and a polarization-velocity-vector calculating device configured to calculate a characteristic vector, which expresses a rate of polarization change in the measurement object, from the time dependency of the state-of-polarization vector, wherein the polarization-velocity-vector calculating device calculates a polarization-velocity-vector outer-product operator vx provided by formula (1) below from a Mueller matrix R(t) of the measurement object and determines a polarization velocity vector v (v 1 , v 2 , v 3 ) from a matrix component that provides the polarization-velocity-vector outer-product operator: v ⇀ x = { ∂ ∂ t R ( t ) } R † ( t ) = [ 0 - v 3 v 2 v 3 0 - v 1 - v 2 v 1 0 ] ( 1 )