The invention relates to a blow-by gas recirculation system for an internal combustion engine. An oil separation cover 10 has a surge-tank mounting portion 400 integrally formed therewith in a columnar shape having a cam-shaped cross-section and a height approximately equal to that of an oil separation space OS. The surge-tank mounting portion 400 has a flat upper surface serving as a mounting seat surface 401 capable of coming into surface contact with an attaching seat surface 7a of a surge tank 7. A first opening 403 of a gas passage from a chamber 300 is opened in a vicinity of an opening of a support through-hole 402 on the mounting seat surface 401. Through an operation of joining the seat surface 7a of the surge tank 7 to the mounting seat surface 401 of the surge-tank mounting portion 400 while communicating the first opening 403 with a second opening 7b opened on the attaching seat surface 7a of the surge tank 7, a third gas passage 15 for supplying blow-by gas to an inside of the surge tank 7 is formed in such a manner as to penetrate through the oil separation cover 10 and the surge tank 7.