A light amount detection device comprising: an amplifier that is adapted to perform amplification processing of amplifying a detection signal from a photomultiplier tube for detecting light; an A/D converter that is adapted to perform A/D conversion processing of converting the detection signal amplified by the amplifier into a digital signal to output the digital signal; a base voltage calculation part that is adapted to perform base voltage calculation processing of, for the output signal from the A/D converter, calculating, as a base voltage, a time average value of signal components each having a voltage lower than a predetermined base threshold value (Vbth); a base correction processing part that is adapted to perform base correction processing of offsetting the output signal from the A/D converter in such a manner that the base voltage calculated by the base voltage calculation part becomes 0; a dark current calculation part that is adapted to perform dark current calculation processing of, for an output signal from the base voltage calculation part in a non-input state in which light is not input into the photomultiplier tube, calculating, as a dark current pulse, a signal component having a voltage higher than a predetermined signal detection threshold value (Vth); a threshold value processing part that is adapted to perform threshold value processing of, for the output signal from the base voltage calculation part, calculating, as a detection light pulse, a signal component having a voltage higher than a predetermined signal detection threshold value (Vth); and a light emission signal amount calculation part that is adapted to perform light emission signal amount calculation processing of calculating a light emission signal amount by subtracting, from the signal component of the detection light obtained by the threshold value processing, a time average value of the signal components of the dark current obtained by the dark current calculation processing. an amplifier that is adapted to perform amplification processing of amplifying a detection signal from a photomultiplier tube for detecting light; an A/D converter that is adapted to perform A/D conversion processing of converting the detection signal amplified by the amplifier into a digital signal to output the digital signal; a base voltage calculation part that is adapted to perform base voltage calculation processing of, for the output signal from the A/D converter, calculating, as a base voltage, a time average value of signal components each having a voltage lower than a predetermined base threshold value (Vbth); a base correction processing part that is adapted to perform base correction processing of offsetting the output signal from the A/D converter in such a manner that the base voltage calculated by the base voltage calculation part becomes 0; a dark current calculation part that is adapted to perform dark current calculation processing of, for an output signal from the base voltage calculation part in a non-input state in which light is not input into the photomultiplier tube, calculating, as a dark current pulse, a signal component having a voltage higher than a predetermined signal detection threshold value (Vth); a threshold value processing part that is adapted to perform threshold value processing of, for the output signal from the base voltage calculation part, calculating, as a detection light pulse, a signal component having a voltage higher than a predetermined signal detection threshold value (Vth); and a light emission signal amount calculation part that is adapted to perform light emission signal amount calculation processing of calculating a light emission signal amount by subtracting, from the signal component of the detection light obtained by the threshold value processing, a time average value of the signal components of the dark current obtained by the dark current calculation processing.