To provide an attachment structure of a vacuum pump that achieves always effectively an excellent breather function and pumping loss reduction.

The inside of an engine case is sucked to turn the pressure in the engine case into a negative pressure. A breather chamber 22 is disposed above the vacuum pump 24. An intake port 44 of a blow-by gas is disposed at an upper part of the vacuum pump 24. A discharge port 45 of the blow-by gas is disposed at a lower part of the vacuum pump 24.