Interstitial photodynamic therapy (iPDT), which uses optical fibers to excite photosensitizers in PDT, is a promising approach in the treatment of internal tumors. To enhance the treatment efficacy, the placement of the inserted optical fibers should be optimized from the photosensitizer distribution in the affected area. Here, we propose a method to obtain a fluorescence distribution of photosensitizers during iPDT. The inserted optical fibers for iPDT can be also used for detection of the fluorescence from the photosensitizer in the tissue. In the proposed method, fluorescence signals from the photosensitizers are collected by changing the combination of the inserted optical fibers for excitation and detection. The detected fluorescence signals are modeled by using excitation light distribution, fluorescence detection efficiency, and photosensitizer distribution. The excitation light distribution and the fluorescence detection efficiency distribution are obtained by calculating light propagation in the tissue. The photosensitizer distribution is reconstructed from the detected signals with a compressive sensing algorithm with a sparsity constraint. As a demonstration, we performed numerical simulations to obtain a photosensitizer distribution in a biological tissue by inserting several optical fibers for excitation and detection. In the numerical simulation, we verified the spatial distribution of the photosensitizers in a biological tissue can be reconstructed from the fluorescence signals detected by inserting the optical fibers.