The present invention provides a therapy-progress-level monitoring device and a method thereof, which make it possible to know, in real time during a therapy, the therapy depth of extracellular photodynamic therapy. The device is a photodynamic therapy progress-level monitoring device (1), and is provided with: an irradiation unit (30) configured to irradiate, with excitation light of a wavelength capable of exciting a photosensitive substance, a photosensitive substance distributed and sustainably supplied by vascular permeability in the extracellular fluid and the transcellular fluid of a living body detection unit (30, 20, 13) configured to detect, during a detection period, the fluorescence intensity produced when the photosensitive substance excited by a reaction between the excitation light and the photosensitive substance is inactivated into a ground state a depth calculation unit (14) configured to calculate in real time an indicator value estimating the volume or the depth of a therapy region by using the fluorescence intensity during the detection period and output unit (14, 16, 17) configured to output the indicator value in real time to a display unit or a storage unit.
