[Object] To provide an estimating apparatus and an estimating method capable of estimating process of therapy using a laser the catheter 300 accurately in real time. [Solving Means] A photodynamic therapy apparatus 1 as an estimating 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 the catheter 300, including a connector 210, a light source 110, and a light detection unit 130. The laser the catheter 300 is capable of being attached/detached to/from the connector 210. The light source 110 outputs the excitation light to the laser the catheter 300 via the connector 210. The light detection unit 130 detects intensity or a spectrum of the fluorescence, the fluorescence being entered from the laser the catheter 300 via the connector 210, to estimate whether the tissue has changed because of reaction between the excitation light emitted from the tip portion of the laser the catheter 300 and the photo-sensitive pharmaceutical absorbed in the tissue.
