PURPOSE: To make it possible to clearly display the adjustment of air-fuel ratio so as to facilitate adjustment of an air-fuel ratio by disposing two voltage comparators which receive an output from O2 sensor and judge whether the air-fuel ratio is excessively rich or lean and lighting an LED by means of said output.

CONSTITUTION: An output (V) from an O2 sensor 1 is compared with comparative voltage (E1) 202 at a voltage comparator 201. When V is larger than E1, an H level signal is outputted from said comparator 201 and a transistor (Tr) 204 is switched ON to light an LED 205. On the other hand, an output (V) from said O2 sensor 1 is compared with a voltage (E2) 302 at a voltage comparator 301. When V is larger than E2, an H level signal is outputted from said comparator 301 and a Tr 304 is switched ON to light an LED 305. Namely, when the air- fuel ratio is excessively rich, only the LED 205 is lighted, and when the ratio is excessively lean, only the LED 305 is lighted, respectively. When the ratio is near the set value, both LED 205, 305 are lighted because of E1<V<E2.