Currently oncological diseases manifest the highest mortality, moreover, almost everyone is susceptible to the appearance of the most dangerous type of cancer - melanoma. One of the effective treatments for skin cancers is photodynamic therapy (PDT) combining light and some Reactive Oxygen Species (ROS) photosensitizers. As a photosensitizer, tetrapyrrole molecules such as chlorin e6 (Сe6) use due to their ability to generate singlet oxygen (SO). However, cancer cells have a pH <7 and a change in the acidity level of the medium (pH <7) leads to aggregation of Ce6 molecules. The effectiveness of Ce6 as SO generators is determined by whether they are in the form of monomers or aggregates in cells. Ce6 is widely used as a drug for PDT, but today there is no direct information on exactly in what state Ce6 is in the cells. Therefore, it now seems relevant to study the optical properties of Ce6 in melanoma cells. For the first time absorption spectra, PL spectra, PL excitation spectra and PL kinetics of Ce6 molecules in melanoma A375 cells were measured and analyzed. We also investigated the optical properties of Ce6 in cells after their irradiation with 650 nm light, which is effectively absorbed by Ce6 resulting in efficient generation of SO. Analysis of optical properties of Ce6 in the cells has demonstrated that the spectral positions of its first absorption band and the PL band centered at 675 nm correspond to Ce6 monomers. We suppose that binding of Ce6 to mitochondria is a reason of monomerization of Ce6 in acidic environment inside melanoma cells. Our results clearly demonstrate that Ce6 is a perfect PDT drug due to preservation of its monomeric form inside the cells, which is characterized by the best SO generation efficiency with respect to Ce6 dimers and other aggregates.