An intake structure for an internal combustion engine, in which: the internal combustion engine (30, 30') having a combustion chamber includes a cylinder block and a cylinder head, the cylinder head having an intake port and an exhaust port defined therein; an inlet pipe is connected to the intake port, defining a continuous intake passageway; a throttle body having an intake channel continuing to the intake passageway is connected to an upstream side of the inlet pipe with respect to an intake flow therein; the throttle body is provided with a throttle valve and a tumble valve disposed downstream thereof with respect to a flow direction of the intake flow in the intake channel; and the intake passageway is divided by a partition plate downstream of the tumble valve into an upper main passageway and a lower auxiliary passageway serving as a tumbling flow channel for producing a tumbling flow in the combustion chamber, wherein an idling air passageway is defined in an upper side of the intake channel to provide fluid communication between a region upstream of the throttle valve and a region downstream of the throttle valve, the idling air passageway having a downstream outlet, the downstream outlet being defined in an upper inner surface of the intake channel between the throttle valve and the tumble valve, and the tumble valve has a tumble valve plate and a recess defined therein, the tumble valve being arranged such that when the throttle valve is fully closed, idling air flowing out of the idling air passageway is restrained by the tumble valve plate from flowing into the upper main passageway and is guided from the recess to the lower auxiliary passageway. the internal combustion engine (30, 30') having a combustion chamber includes a cylinder block and a cylinder head, the cylinder head having an intake port and an exhaust port defined therein; an inlet pipe is connected to the intake port, defining a continuous intake passageway; a throttle body having an intake channel continuing to the intake passageway is connected to an upstream side of the inlet pipe with respect to an intake flow therein; the throttle body is provided with a throttle valve and a tumble valve disposed downstream thereof with respect to a flow direction of the intake flow in the intake channel; and the intake passageway is divided by a partition plate downstream of the tumble valve into an upper main passageway and a lower auxiliary passageway serving as a tumbling flow channel for producing a tumbling flow in the combustion chamber, wherein an idling air passageway is defined in an upper side of the intake channel to provide fluid communication between a region upstream of the throttle valve and a region downstream of the throttle valve, the idling air passageway having a downstream outlet, the downstream outlet being defined in an upper inner surface of the intake channel between the throttle valve and the tumble valve, and the tumble valve has a tumble valve plate and a recess defined therein, the tumble valve being arranged such that when the throttle valve is fully closed, idling air flowing out of the idling air passageway is restrained by the tumble valve plate from flowing into the upper main passageway and is guided from the recess to the lower auxiliary passageway.