To provide a method and a chemical capable of readily and inexpensively inhibiting proliferation of cells of human and animals.

The method for inhibiting cell proliferation includes holding a cell in the presence of at least one substance selected from 5-aminolevulinic acids and their salts and nanoparticles containing at least one substance selected from metals and metal compounds. The inhibitor for cell proliferation includes at least one substance selected from 5-aminolevulinic acids and their salts, and nanoparticles containing at least one substance selected from metals and metal compounds. Inhibition of cell proliferation can be carried out readily and inexpensively than conventional method without requiring irradiation with light and an expensive light-emitting apparatus, because inhibition of cell proliferation is performed even when keeping the cell in a dark place and inhibition of cell proliferation can be carried out without light irradiation.