To provide a light measuring device and method capable of measuring the amplitude variation amount, phase variation amount, and time variation of a light signal without using local light emission.

The light measuring device includes: a light branching element for branching light to be measured into a plurality lights; a time delay processing unit for applying predetermined time delay to one of the branched light to be measured; a light phase diversity circuit that sets, as reference light, the other of the branched light to be measured or the light to be measured having undergone the processing by the time delay processing unit, and outputs the same-phase signal component and an orthogonal signal component of the light to be measured due to the interference of the light to be measured and the reference light whose relative time difference is a time applied by the time delay; a data processing circuit for calculating at least one of the amplitude variation amount and phase variation amount; and an optical time gate processing unit or electric time gate processing unit for performing extraction while shifting the phase every predetermined bit period. The light measuring method measures the time variation of the amplitude phase distribution.