In an apparatus (and a method) for controlling a cogeneration system equipped with a generation unit having a generator connectable to an AC power feed line between a commercial power network and an electrical load and an internal combustion engine driving the generator, and a heat exchanger that exchanges heat between coolant and exhaust heat from the engine, there are provided with a condensate water hose discharging condensate water generated by condensation of moisture in exhaust gas, a hose temperature sensor detecting temperature in the hose, an exhaust gas temperature sensor detecting temperature of the exhaust gas from the engine and an exhaust gas blowout discriminator discriminating that the exhaust gas blows out through the hose when a difference between the hose temperature and the exhaust gas temperature is found to be less than a predetermined value. With this, exhaust gas leakage caused by blowout can be easily detected.