To provide a sizing agent for a fiber, capable of maintaining a chopped strand shape, having high affinity with both of a polyolefin of a matrix resin and the fiber, and providing a composite material having sufficient flexural strength.

The sizing agent for the fiber for reinforcing an olefinic thermoplastic resin is obtained by dissolving or dispersing a polyolefin resin (A) of a polymer containing propylene as an essential component and having one or more amino groups in the molecule, and an elastomer (B) having the glass transition temperature of from -150°C to 25°C in an aqueous medium (C).