To facilitate adjustment of a method or device for improving fuel economy by dispersing minute waterdrops in fuel.

Steam is produced to obtain minute waterdrops, and in its cooling process, the steam is uniformly dispersed to form minute waterdrops. The minute waterdrops are supplied together with fuel to cause steam explosion. In regard to the amount of the minute waterdrops to be sucked, a weight ratio of water to fuel is within a range of 0.1-5%. At start of suction of the minute waterdrops, 20 mmHg of intake manifold inner negative pressure is detected, and a valve mounted to a steam supply pipe is turned off. At stop of the suction of the minute waterdrops, 20 mmHg or more of manifold inner pressure is detected and the valve is turned on. A steam generation part of a steam pipe for supplying minute waterdrops to fuel and a steam supply part (steam suction part) of the steam pipe are arranged on an exhaust passage and an intake passage, respectively. A heating part of the steam pipe for supplying minute waterdrops to fuel of the internal combustion engine is arranged on an exhaust passage of an exhaust manifold, and the steam supply part (steam suction part) of the steam pipe is arranged on an intake passage of an intake manifold.