In a general-purpose engine having a throttle valve installed in an air intake passage connected to a combustion chamber, sucked air mixing with fuel to generate an air-fuel mixture to be ignited to drive a piston to rotate a crankshaft connected to a load, a first warm-up time period during which the engine is warmed up and a second warm-up time period which is longer than the first time period are determined based on detected engine temperature and a fuel quantity is increased during the first time period. The operation of the motor is controlled such that a change rate of throttle valve opening is limited within a range until the measured time exceeds the second warm-up time period after it exceeded the first time period. With this, it becomes possible to complete warm-up operation in a short period of time, while improving rate of fuel consumption and emission performance.