Intake system of internal combustion engine

An internal combustion engine includes an intake port (5) leading to an engine cylinder (2). The intake port is divided into first and second passage section (5A,5B) by a partition (11) extending in the intake port in a longitudinal direction of the intake port. A gas motion control valve (31) is arranged to open and close an upstream end of the second passage section. A connection passage (12,12') connects an upstream end portion of the second passage section (5B) to the first passage section (5A). A bulge (51,52) is formed in a first part of a circumferential region surrounding the downstream port end (43) so as to prevent air from being drawn from the cylinder into the second passage section (5B).