An electronic control device (ECU) (50) controls a flow path changing valve (64) in accordance with the operation state of an engine (1), to control the temperature of intake air introduced into the engine (1), by selectively causing external air from an external air inlet (4a), high-temperature air from a high-temperature air passage (63), or mixed air comprising the external air and the high-temperature air, to flow towards the downstream side of an air intake passage (4). The ECU (50) calculates the MBT ignition timing and the knock limit ignition timing, on the basis of the detection results of an intake air temperature sensor (47), a rotational speed sensor (44), and an intake air pressure sensor (48), and controls the flow path changing valve (64) such that, in cases when the knock limit ignition timing is at a more advanced angle than the MBT ignition timing, the high-temperature air or the mixed air is introduced into the engine (1) as the intake air, and, in cases when the knock limit ignition timing is the same as the MBT ignition timing or at a more retarded angle than the MBT ignition timing, the external air is introduced into the engine (1) as the intake air.