PROBLEM TO BE SOLVED: To properly observe a subject even during treatment using PDT (Photodynamic Therapy), while keeping the treatment time as short as possible.SOLUTION: An electronic endoscope system 11 includes a light projection unit 55B, an LD 62 for excitation light, an LD 63 for treatment light, a CCD 21, and a CPU 41. The light projection unit 55B applies light, which is guided from the LD 62 for excitation light, as the excitation light for PDD (Photodynamic Diagnosis) into the subject, and applies light, which is guided from the LD 63 for the treatment light, as the treatment light for the PDT into the subject. The CCD 21 takes an image of the inside of the subject by photoelectrically converting incident light from the inside of the subject. When simultaneously performing the treatment of tumor tissue for being irradiated with the treatment light and photographing using fluorescent light generated during excitation light irradiation, the CPU 41 performs the treatment and the photographing by controlling respective sections so that an irradiation time of the treatment light and an accumulation period of a signal charge can be included in one frame without overlapping each other.
