PURPOSE: To enlarge the amplitude of an intake pressure wave that arrives in an enlarged chamber to raise the resonance effect so as to improve the charging efficiency of an engine by providing the enlarged chamber in the smaller sectional area of an intake passage which is made to have sectional areas of large and small ones in its midway.

CONSTITUTION: An intake mechanism 3 has plural branch suction passage 5 connected to plural intake port 4 respectively opened on plural cylinder 2 of an engine body 1, a common intake passage 6 connected alternately to each branch intake passage 5 and an intake resonator (enlarged chamber) 7 connected to the common intake passage 6 through a connecting passage 8. With this constitution, the sectional area of the common suction passage 6 is made to change at a midway 6a to be larger at an upstream side 6b and smaller at a downstream side 6c. The intake resonator 7 is to be provided on the downstream side 6c where the sectional area of the common intake passage 6 is small. This contrivance makes the amplitude of the intake pressure wave that arrives in the suction resonate 7 larger to raise the resonance effect so as to improve the charging efficiency of an engine.