To provide a light irradiation method using a pulse laser applicable to the technology of PDD and / or PDT, and a tumor treatment system having a higher tumor killing effect. Provided is a light irradiation method for irradiating a cell into which a photosensitive compound capable of generating singlet oxygen is incorporated with a pulse laser having a wavelength included in the Soret band. The pulse width of the pulse laser can be 100 psec or less, the wavelength can be 405 ± 10 nm, and the photosensitive compound capable of generating singlet oxygen is a photodynamic therapy (PDT) drug or photodynamic diagnosis (PDD). Drugs for use can be used.
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