To reliably separate blow-by gas from oil mist while suppressing an increase in the size and weight of an internal combustion engine.

This gas-liquid separation oil return passage has a double-hole structured path which is communicated at one end with a crank case 52 and communicated at the other end with an intake passage 57 and which has an outside hole as an oil return path W2 and an inside hole as a blow-by gas reduction path W1. It comprises a gas-liquid separating means provided in the blow-by gas reduction path W1 for separating oil mixed in blow-by gas. For example, the used gas-liquid separating means is a foamed member 3 which has a plurality of cells 3a and whose outer peripheral face forms a boundary between the oil return path W2 and the blow-by gas reduction path W1.