A catalyzer control apparatus for purifying exhaust gas from an internal combustion engine has an electrically heated catalyzer that is heated by a battery which battery is isolated from the conventionally provided engine battery. The temperature of the catalyzer is detected by a sensor and a control circuit located with an engine control unit selectively applies power to heat the catalyzer. The catalyzer is arranged to be pre-heated prior to engine start and by isolating the battery for heating the catalyzer from the normally provided engine battery so, because the catalyzer takes a great power, engine starting is enhanced by virtue of having separate batteries.