An optical measurement instrument for a living body, comprising: a probe unit comprising plural irradiation parts (1011, 2011) for irradiating light onto a subject, and a light detection part for detecting light transmitted through or reflected from the subject; and a processing and control unit for receiving a received light signal from said probe unit and acquiring a living body signal; wherein said probe unit is adapted to sequentially distribute a driving signal to the irradiation parts so that the timings of light irradiation by the light irradiation parts do not overlap, said processing and control unit is adapted to transmit a light modulation signal to said probe unit, characterised in that said probe unit comprises a first driving device for receiving said light modulation signal and emitting a first driving signal, and a second driving device for receiving said light modulation signal and emitting a second driving signal, the first and second driving signals emitted from the first and second driving devices (1014, 2014) being rectangular waves modulated by the same frequency but with their phases inverted relative to each other, said probe unit further comprises a first multiplexer for sequentially distributing said first driving signal to each of a first plurality of said irradiation parts, and a second multiplexer for sequentially distributing said second driving signal to each of a second plurality of said irradiation parts, so that the light emitted by each of the first and second plurality of irradiation parts (1011, 2011) during its timing of light irradiation is intensity modulated at the modulation frequency which is the same for each irradiation part, and the processing and control unit is adapted to subject the received light signal to lock-in detection processing. a probe unit comprising plural irradiation parts (1011, 2011) for irradiating light onto a subject, and a light detection part for detecting light transmitted through or reflected from the subject; and a processing and control unit for receiving a received light signal from said probe unit and acquiring a living body signal; wherein said probe unit is adapted to sequentially distribute a driving signal to the irradiation parts so that the timings of light irradiation by the light irradiation parts do not overlap, said processing and control unit is adapted to transmit a light modulation signal to said probe unit, characterised in that said probe unit comprises a first driving device for receiving said light modulation signal and emitting a first driving signal, and a second driving device for receiving said light modulation signal and emitting a second driving signal, the first and second driving signals emitted from the first and second driving devices (1014, 2014) being rectangular waves modulated by the same frequency but with their phases inverted relative to each other, said probe unit further comprises a first multiplexer for sequentially distributing said first driving signal to each of a first plurality of said irradiation parts, and a second multiplexer for sequentially distributing said second driving signal to each of a second plurality of said irradiation parts, so that the light emitted by each of the first and second plurality of irradiation parts (1011, 2011) during its timing of light irradiation is intensity modulated at the modulation frequency which is the same for each irradiation part, and the processing and control unit is adapted to subject the received light signal to lock-in detection processing.