We studied photosensitization reaction progress in a cell culture well by oxygen partial pressure distribution measurement along the well depth direction with a high concentration of talaporfin sodium solution. The talaporfin sodium solution of 20 μg/ml in concentration with 2.8 mm thickness in the well was irradiated from the well bottom by 663 nm excitation laser with 0.29 W/cm 2. A small Clark-type oxygen electrode was used to measure oxygen partial pressure during the photosensitization reaction with approximately 200 μm resolution. Corrections against solution temperature change and direct light irradiation were applied to the electrode output. The oxygen partial pressures at various depths were decreased uniformly from the atmospheric oxygen partial pressure with the photosensitization reaction progress up to the irradiation of 1.0 J/cm2 in radiant exposure. In the case of photosensitization reaction over 1.0 J/cm2 in radiant exposure, the oxygen partial pressure distribution along the well depth was non-uniform. In the case of photosensitization reaction with 40 J/cm2 in radiant exposure in the solution without cells, there was pressure gradient of 2.8×104 mmHg/m from 0.5 to 1.0 mm in depth from the solution surface. In this case, there was no pressure gradient near the bottom of the well. In contrast, with myocardial cells at the bottom, oxygen partial pressure gradient of 7.5×103 mmHg/m from 1.5 to 2.0 mm in the depth was appeared after irradiation with 40 J/cm2 in radiant exposure. Consequently, we found that oxygen partial pressure distribution along the depth in the well with high concentration of talaporfin sodium solution was dynamically changed with time of the photosensitization reaction using the laser irradiation from the bottom. We think this dynamic pressure change in the well might be useful to understand the photosensitization reaction progress in the well experiment system in vitro corresponding to the extracellular PDT.