An air intake device for an engine includes a duct having therein a first air flow passage, a housing having therein a second air flow passage, a rotary valve having a valve plate. A block of the housing, which conducts intake air to an intake port of a cylinder head, has a circular opposed surface that follows a shape of a curved surface portion of the valve plate. Moreover, the block of the housing has a protrusion that protrudes toward an upstream side of the second air flow passage in an air flow direction so as to contact a duct protrusion of the duct. Therefore, a localized flow of the intake air, which has flowed into the second air flow passage from an opening of the first air flow passage, can be restrained from spreading, and a flow rate of the localized flow can be restrained from slowing down.