[Object] To provide a calculation apparatus and a calculation method capable of calculating pharmaceutical concentration in a tissue in real time. [Solving Means] A photodynamic therapy apparatus 1 as a calculation apparatus is an apparatus for irradiating a tissue having absorbed photo-sensitive pharmaceutical, the photo-sensitive pharmaceutical absorbing an excitation light and emitting fluorescence, with the excitation light emitted from a tip portion of a laser catheter 300, including a connector 210, a light source 110, and a light detection unit 130. The laser catheter 300 is capable of being attached/detached to/from the connector 210. The light source 110 outputs the excitation light to the laser catheter 300 via the connector 210. The light detection unit 130 detects intensity of the fluorescence, the fluorescence being entered from the laser catheter 300 via the connector 210, to calculate concentration of the photo-sensitive pharmaceutical in a tissue, the tip portion of the laser catheter 300 contacting the tissue.
