To perform smooth accelerating operation at accelerating operation time at which a throttle valve is opened just after starting operation.

In a start valve guide cylinder 18, a start fuel passage 19 connected to a fuel source, start mixture passage 21 connected to an intake path 2A in an engine side from a throttle valve 3 and a start air passage 20 are opened. Opening of the passages 19, 20, 21 to the start valve guide cylinder 18 is controlled to be opened/closed by a start valve 22. An upstream opening 20A of the start air passage 20 is opened to an intake path 2B in an air cleaner side. An upstream opening 23A of an auxiliary air passage 23 branching from the start air passage 20 is opened into a throttle valve guide cylinder 2C, also closed by the throttle valve 3 in its fixed opening or less, to be opened in the fixed opening or more.