PURPOSE: To realize good atomization of fuel at the time of high load low speed operation with a simplified structure by press-feeding air in the downstream part of an assist air supply path synchronized with injection action when an engine speed is in a predetermined value or less and an intake pipe pressure is in a predetermined value or more.

CONSTITUTION: When an engine is in low load operation, a sufficient differential pressure is generated between the upstream and downstream of a throttle valve 1 with a small opening thereof, and sufficient assist air is supplied in the vicinity of a fuel injection valve F by opening a check valve 33 to atomize injected fuel. When the engine is in high load low speed operation, the throttle valve 1 is large opened to generate a small differential pressure between the upstream and downstream of the throttle valve 1, and no assist air is supplied by closing the check valve 33, but when the fuel injection valve F is injection-operated, a piston 32 is actuated synchronously with fuel injection by a piston driving means 4 to press-feed air into a downstream part 21 of an assist air supply path 2 and to become the assist air supplied in the vicinity of the fuel injection valve F. Thus, supply of the assist air is improved at fuel injection time through a total region of high load low speed operation.