For safe and effective interstitial PDT (iPDT), treatment planning is required since the efficacy depends on optical irradiation conditions including the number of inserted optical diffusers, the insertion positions, their powers, and so on. We are developing a treatment planning method based on a numerical model of singlet oxygen generation during light irradiation for iPDT. In the model, photosensitizer photobleaching, which occurs usually during light irradiation for iPDT, is taken into consideration. In this paper, we implement a numerical simulation of the efficacy of iPDT of malignant brain tumor with 5ALA according to desired optical irradiation conditions and evaluate quantitatively the photobleaching effect on the efficacy simulation. In the simulation, to calculate light propagation into biotissues, a 3D brain model segmented into tissue types, brain normal and tumor tissues, was constructed using an MRI data of a patient with a brain tumor (tumor volume: 34 cm3). Supposing that 8 optical diffusers (light power density of the diffuser surface: 580 mW/cm2, length: 40 mm, diameter: 1.1 mm) were inserted into the tumor region, the light distribution was calculated. Then, the photobleached ALA and the accumulated singlet oxygen distribution were calculated to obtain the treated volumes depending on singlet-oxygen concentration threshold values to induce cell death. Compared to the estimated treatment volume without the photobleaching effect, the estimated efficacy with the proposed model was decreased. This result indicates that the model with photobleaching effect will lead to improvement of the efficacy prediction of iPDT.