An apparatus for circulating a blowby gas to engine cylinders in an internal combustion engine, including a cylinder head cover including a first wall defining a first passage, and a first flange outwardly extending from the first wall, and a cylinder head including a second wall and a second flange cooperating with the first flange to form an abutting surface therebetween on which a second passage is arranged. The second passage is connected with the first passage and extending in a direction of a row of the engine cylinders. The second wall defines a plurality of third passages each having one end that is open to the second flange and communicated with the second passage and an opposite end open to an intake port of each of the engine cylinders.