A photodynamic therapy apparatus 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 catheter, including a connector, a light source, and a light detection unit. The laser catheter is capable of being attached to and detached from the connector. The light source outputs the excitation light to the laser catheter via the connector. The light detection unit detects intensity or a spectrum of the fluorescence, the fluorescence being entered from the laser catheter via the connector, to estimate whether the tissue has changed because of reaction between the excitation light emitted from the tip portion of the laser catheter and the photo-sensitive pharmaceutical absorbed in the tissue.