Photodynamic therapy (PDT) in the respiratory field requires appropriate light irradiation in a confined space. In silico evaluation based on computer simulation of light propagation in tissues is effective for analyzing the light irradiation conditions. Focusing on PDT for peripheral lung cancer, this paper reviews a computer simulation method of light propagation in tissue by a Monte Carlo method, evaluation of PDT light irradiation probes, and pretreatment clinical evaluation of PDT light irradiation conditions using clinical image data.