This composition contains a polymer (A), a monomer (B) and a polymerization initiator (C). This composition contains 100-280 parts by mass of the polymer (A) per 100 parts by mass of the monomer (B), and the polymer (A) contains 70% by mass or more of a methyl methacrylate unit. The monomer (B) contains 70-99.5% by mass of at least one compound that is selected from among compounds (b1) represented by formula (1) and methyl methacrylate (b2), and 0.5-30% by mass of 1,4-pentadien-3-ol (b3). A molded article that has excellent toughness and low water absorption, while exhibiting high strength and high elastic modulus can be obtained using this composition. (In the formula, each of R1 and R2 independently represents a hydrogen atom or a methyl group, and at least one of R1 and R2 is a hydrogen atom.)
