To provide a light measurement device that can surely detect dental plaque of a small amount, and is downsized enough to be capable of being easily assembled into the toothbrush, and is simply configured.SOLUTION: A light measurement device has: a first light source that emits light in a first light emitting period within a first cycle of a reference signal, and exits light of a first wavelength； a second light source that emits light in a second light emitting period not overlapping the first light emitting period within the first cycle of the reference signal, and exits light of a second wavelength longer in a wavelength than the first wavelength； a detection unit that detects first and second fluorescence intensity generating when irradiating a sample with the light of the first and second wavelengths； a setting circuit that sets up lengths of the first and second light emitting periods； first and second phase detectors that conduct phase detection according to the reference signal and a signal having the reference signal delayed, and generate first and second output signals in accordance with the first and second fluorescence intensity； and a control circuit that calculates an amount of fluorescent substance of a measurement object by a computation using the first and second output signals.SELECTED DRAWING: Figure 8【課題】少量の歯垢を確実に検出でき、しかも歯ブラシに容易に組み込むことができる小型でシンプルな構成の光測定装置を提供する。【解決手段】光測定装置は、基準信号の１周期内における第１の発光期間に発光して、第１の波長の光を出射する第１の光源と、基準信号の１周期内における第１の発光期間と重複しない第２の発光期間に発光して、第１の波長よりも長波長である第２の波長の光を出射する第２の光源と、試料に第１及び第２の波長の光を照射したときに発生する第１及び第２の蛍光強度を検出する検出部と、第１及び第２の発光期間の長さを設定する設定回路と、基準信号及び基準信号を遅延させた信号に従い位相検波を行って、第１及び第２の蛍光強度に応じた第１及び第２の出力信号を生成する第１及び第２の位相検波器と、第１及び第２の出力信号を用いた演算により測定対象の蛍光物質量を算出する制御回路とを有する。【選択図】図８
