A polymerization curable composition comprising a specific polyfunctional polymerizable monomer having a hard molecular structure with an L-scale Rockwell hardness of its homopolymer of 60 or more, a specific polyfunctional polymerizable monomer and a photochromic compound, a photochromic lens substrate composed of a cured product of the polymerization curable composition, and a lens comprising the photochromic lens substrate.The substrate and lens show excellent photochromic properties such as high color development density and high fading rate, and the cured product has excellent substrate properties such as hardness, heat resistance and impact resistance as well as strength (toughness) that allows for its use in rimless spectacles.