An engine electronic control unit is inserted through a through hole provided in an intake pipe and mounted in an intake air passage in a direction substantially perpendicularly with respect to a plane of the intake pipe forming the intake air passage. This unit is then secured to the intake pipe using a fixing flange provided at a connector portion. A fixing rail is protruded inside the intake pipe and leading edges of a metal base and a metal cover of the unit are inserted into this rail, thereby securing in position an end opposite to a side of the connector portion of the unit. This realizes an engine electronic control unit offering an outstanding heat radiation performance and vibration resistance, without having to provide special heat radiating parts or without involving an increase in an intake air resistance within the intake air passage. By using such an engine electronic control unit, it is possible to provide a low-cost, compact engine air intake system.