PURPOSE: To facilitate tuning of performance of an internal combustion engine by providing plural pairs of runners having a configuration spirally revolving from a starting point, a central plenum zone into which air is introduced from an intake part, in an air induction system connected with an intake hole in the internal combustion engine.

CONSTITUTION: This air induction system 12 is connected with an intake hole 16 provided in a cylinder head 14 of a four-air-cylinder internal combustion engine 10 through a rail block 18 having same number of fuel injectors 20 as the number of the air cylinders. This air induction system 12 is constituted to have four pairs of spiral runners for connecting a central plenum zone 52 with facing cylinders of the engine 10. Each pair of runners is formed from a long and slender runner 58 and a short and thick runner 60, inlets 62, 64 of respective runners 58, 60 are communicated with the plenum zone 52, and outlets 66, 68 are connected with corresponding inlets 70, 72 in the rail block 18. Each of runners 58, 60 is formed so that its circumferential extent exceeds 360 degrees from a starting part.