To provide a blow-by gas treatment device for an internal combustion engine capable of providing the blow-by gas treatment device for the internal combustion engine with a simple constitution capable of purifying the blow-by gas and releasing it to atmosphere without refluxing it to an intake system.

A by-pass passage (24) having a throttling part (26) is formed on an exhaust pipe (14) of the engine (1), and a blow-by gas passage (28) extending from the engine is communicated with the throttling part. An exhaust post-treatment device (18) internally attached with an oxidation catalyst (20) and DPF (22) is provided on an exhaust downstream side of the by-pass passage.