Fuel gas mixer

A fuel gas mixer and a method for controlling a fuel gas mixer comprising a throttle valve (4) provided for opening and closing an air intake passage (2) of an engine, and a fuel gas supply passage (6) connected to said air intake passage at a position upstream of said throttle valve (4). A fuel gas control valve (5) for opening and closing said air intake passage (6) is provided in said air intake passage at a position upstream of the position at which said fuel gas supply passage (6) is connected to said air intake passage. A fuel gas control valve controlling means (13,14) is provided for varying the degree of opening of said fuel gas control valve (5) according to the operating state of said engine. Said fuel gas control valve controlling means (13,14) comprises an electronic control unit (13) and a step motor (14) for opening and closing said fuel gas control valve (5).