To provide an arranging structure of a separator for blow-by gas, capable of preventing an excessive temperature rise in the blow-by gas, and capable of preventing freezing of moisture, in the separator for the blow-by gas.

Excessive temperature rise in blow-by gas in a PCV separator 2 can be prevented by slow heat transmission via a void 20a, and freezing of moisture in the blow-by gas can be prevented. Since a heat insulation material 20 is a resin sponge and in a compressed state, an abnormal sound and friction is prevented and its retentivity is enhanced. Since the excessive temperature rise is not caused in the PCV separator 2, even if the blow-by gas is discharged to an intake passage, heat damage is not caused. Since moisture freezing is not generated, a flow of the blow-by gas and separated oil is not hindered in the PCV separator 2. Thus, there is no such thing as removing performance of oil mist in the blow-by gas is lowered, it becomes difficult to discharge the separated oil, or the blow-by gas cannot be smoothly discharged to the intake passage.