To provide a blow-by gas treatment device capable of achieving both of reduction of a consumption amount of oil caused by blow-by gas and reduction of a generation amount of a deposit caused by the blow-by gas.

A blow-by gas treatment device 50 includes gas introduction means 51-54 for introducing blow-by gas (black arrow) to an intake passage 31 of an engine 10, and an oil separator 55 which is an oil separating means for separating at least a part of oil particles from the blow-by gas, in which treatment is carried out so that a degree of separation of large oil particles is larger than a degree of separation of small oil particles. The gas introduction means is constituted so as to introduce both of first gas (passing through a passage part 51) which is the blow-by gas before being treated by the oil separating means and second gas (passing through a passage part 52) which is the blow-by gas after being treated by the oil separating means to the intake passage.