An air intake device of an engine includes a throttle body provided in an intake passage of an engine, a bypass passage arranged to connect an upstream side and a downstream side of a throttle valve disposed in the throttle body, an idle number-of-revolutions control device that is disposed in the bypass passage and that controls the quantity of air passing through the bypass passage and flowing into the downstream side of the throttle valve according to the state of the engine, and a water collection portion that is disposed on the upstream side of the idle number-of-revolutions control device in the bypass passage and that is constructed so as to include a space portion connecting with the bypass passage. This structure enables smooth operation of the idle number-of-revolutions control device and prevents damage thereto caused by water in a bypass passage.