A photodetecting device 1 includes an integrating sphere 20 for observing light to be measured generated according to irradiation of a sample with excitation light and a sample holder 60 removably attached to the integrating sphere 20, the integrating sphere 20 has an excitation light introducing hole 201 for introducing the excitation light and a sample introducing hole 205 for introducing a cell C held by the sample holder 60, the sample holder 60 is locked to the sample introducing hole 205 and holds the cell C for accommodating the sample, and the cell is disposed so that an entrance surface of the cell C, through which the excitation light enters the cell C, inclines relative to the surface perpendicular to the optical axis L of the excitation light.