A switching mechanism of an air circuit breaker that allows the overall size of the air circuit breaker to be minimal by making the width of the switching mechanism relatively small. The switching mechanism of the air circuit breaker includes both side plates, a main shaft, a main shaft lever, a first link, a closing spring unit, a charging cam, a rotational shaft of the charging cam, a third link, a driving lever, a spring support pin and an end portion supported by the spring support pin, and a third link elastic bias spring having an end portion supported by the spring support pin and an operation portion for providing elastic force onto one surface of the third link to bias the third link to be rotated in one direction. A third link elastic bias spring directly provides the elastic force onto the third link in order to minimize the distance between a pair of driving levers and thus minimize the width of the switching mechanism.