A blow-by gas recirculation structure for an engine includes a gas-liquid separating member, a cylinder head, a support member and a mounting bolt. The gas-liquid separating member includes a gas-liquid separating passage to separate oil from the blow-by gas. The cylinder head includes a communication passage communicating with the air intake system. The support member is configured and arranged to rotatably support a camshaft in the cylinder head. The mounting bolt securely fastens the support member to the cylinder head with the mounting bolt threadedly engaging within the communication passage of the cylinder head. The mounting bolt has an axial through hole axially extending there-through and communicating with the gas-liquid separating passage so that the blow-by gas that has passed through the gas-liquid separating passage is recirculated from the axial through hole of the mounting bolt, through the communication passage, and into the air intake system.