PURPOSE:To make a controller conformable to a yet wider range of a fuel gas constituent and a driving condition, by keeping an air-fuel ratio at an oxygen sensor part in an exhaust system controlling the opening of a gas flow control valve in a gas feed route, in an almost lambda=1 range and thereby letting it perform exhaust gas emission control by a catalyzer so favorably. CONSTITUTION:According to output of the lambda type oxygen sensor 6 installed in an exhaust system of an engine 4, opening of a gas flow control valve 2 inside a gas feed route 1 is controlled by a control part 8 and thereby an air-fuel ratio at a portion of the oxygen sensor 6 is kept in an almost lambda=1 range, whereby exhaust gas emission control by means of a ternary catalyzer 7 is made to be done so favorably. At the above-mentioned, as keeping the opening of the gas flow control valve 2 at the rich or lean side, it is driven so as to shift it to the opposite side beyond the opening corresponding to the lambda=1 range at every fixed period and to be restored to its original position immediately. And, a ratio between the time that the detected value of the oxygen sensor 6 becomes more than a threshold value and the time that it becomes less than that is calculated, and the shift value is compensated according to a deviation with the desired ratio.
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