To enable measurement over a wide dynamic range from weak light quantity to strong light quantity in a light quantity detection device for detecting the light quantity, a detection signal from a photon counting light detector is A/D converted. When the A/D converted detection signal has a preset threshold value or more, the detection signal is transmitted as it is to a number-of-photons calculation circuit in a subsequent stage, and when the detection signal has the threshold value or less, threshold value processing for transmitting a preset reference value to the subsequent stage is performed. In the number-of-photons calculation circuit, the number of photons or the light quantity incident on the photon counting light detector is acquired from the dimension of an acquired detection signal waveform until the light quantity measurement ends.