Abstract: A breather chamber structure of an internal combustion engine in which condensation of vapor within the breather chamber is prevented, the number of required parts is small, the space efficiency is superior and enlargement of the whole engine can be avoided is provided. In an internal combustion engine having auxiliary machinery attached to a side wall of a cylinder block (3) by means of an auxiliary machinery bracket (10), a breather chamber (20) is formed by the side wall of the cylinder block (3) and the auxiliary machinery bracket (10) between the side wall and the bracket, and a cooling water passage (40), (41) is formed on at least one of the side wall of the cylinder block (3) and the auxiliary machinery bracket (10) swelling in the breather chamber (20).