It is intended to provide a therapeutic and/or diagnostic agent that can be used in photodynamic therapy (PDT) or photodynamic diagnosis (PDD) capable of utilizing infrared-spectrum light such as near-infrared light (NIR), infrared light, or far-infrared light, which attains a deep body penetration. The present invention provides a photodynamic therapeutic or diagnostic agent or a photodynamic therapeutic or diagnostic kit for cancer or infectious disease, comprising: a particle (e.g., a lanthanide particle) that emits upconversion luminescence by infrared-spectrum light such as near-infrared light having a wavelength of 0.7 μm to 2.5 μm; and a photosensitizer (e.g., porphyrin) or a 5-aminolevulinic acid group.