A rotary-type throttling device for an internal combustion engine, comprising: a rotary valve including a valve body having a cylindrical body with a longitudinal center axis (Lc) about which the cylindrical body is swingable between open and closed positions, and an intake hole defined in the cylindrical body and extending perpendicularly across the center axis (Lc); and a throttle body having an intake passage defined therein which is held in fluid communication with an intake channel in the internal combustion engine and a cylindrical valve chamber defined therein to extend perpendicularly across the intake passage, the cylindrical valve chamber receiving therein said cylindrical valve body to be rotatable relative to the cylindrical valve chamber; wherein the valve body of the rotary valve is rotatable between an open position in which the intake hole of the rotary valve is brought into fluid communication with the intake passage of the throttle body and a closed position in which an outer circumferential surface of the cylindrical valve body closes the intake passage; said throttle body has an upstream auxiliary intake passageway defined therein, having an inlet port held in fluid communication with the atmosphere; said cylindrical valve body has a downstream outer circumferential surface and a downstream auxiliary intake passageway defined therein, the downstream auxiliary intake passageway having an outlet port opening at the downstream outer circumferential surface; and said upstream auxiliary intake passageway and said downstream auxiliary intake passageway have a body-side joint fluid communication port and a valve-side joint fluid communication port defined in respective sliding surfaces of said throttle body and said cylindrical valve body, said body-side joint fluid communication port and said valve-side joint fluid communication port being arranged to be capable of overlapping each other for fluid communication with each other; characterised in that: said valve-side joint fluid communication port is formed in an axial side surface of said cylindrical valve body, and said body-side joint fluid communication port is formed in an inner side surface of said tubular valve chamber of the throttle body which is held in sliding contact with said axial side surface. a rotary valve including a valve body having a cylindrical body with a longitudinal center axis (Lc) about which the cylindrical body is swingable between open and closed positions, and an intake hole defined in the cylindrical body and extending perpendicularly across the center axis (Lc); and a throttle body having an intake passage defined therein which is held in fluid communication with an intake channel in the internal combustion engine and a cylindrical valve chamber defined therein to extend perpendicularly across the intake passage, the cylindrical valve chamber receiving therein said cylindrical valve body to be rotatable relative to the cylindrical valve chamber; wherein the valve body of the rotary valve is rotatable between an open position in which the intake hole of the rotary valve is brought into fluid communication with the intake passage of the throttle body and a closed position in which an outer circumferential surface of the cylindrical valve body closes the intake passage; said throttle body has an upstream auxiliary intake passageway defined therein, having an inlet port held in fluid communication with the atmosphere; said cylindrical valve body has a downstream outer circumferential surface and a downstream auxiliary intake passageway defined therein, the downstream auxiliary intake passageway having an outlet port opening at the downstream outer circumferential surface; and said upstream auxiliary intake passageway and said downstream auxiliary intake passageway have a body-side joint fluid communication port and a valve-side joint fluid communication port defined in respective sliding surfaces of said throttle body and said cylindrical valve body, said body-side joint fluid communication port and said valve-side joint fluid communication port being arranged to be capable of overlapping each other for fluid communication with each other; characterised in that: said valve-side joint fluid communication port is formed in an axial side surface of said cylindrical valve body, and said body-side joint fluid communication port is formed in an inner side surface of said tubular valve chamber of the throttle body which is held in sliding contact with said axial side surface.