A fuel vapor treating mechanism includes a canister for adsorbing fuel vapor produced in a fuel feed system. The treating mechanism purges the fuel vapor adsorbed by the canister, along with air, to an intake system of the engine. An ECU computes a value representing the flow rate of the purged gas as a value that represents the capability of the treating apparatus. The ECU sets a decision value in accordance with the amount of the fuel vapor produced in the fuel feed system. The decision value represents a required capability of the treating mechanism. When the value representing the capability is less than the decision value, the ECU prohibits stratified charge combustion and causes the engine to perform homogeneous charge combustion. Therefore, as many opportunities as possible are provided to perform stratified charge combustion, which improves fuel efficiency.