Water to gas oil (W/O) emulsified diesel fuel has been researched actively to reduce the load on the environment. However, this is due to the large drop-size distribution and the polydispersion W/O emulsion produced by an agitation-type method. We examined the characteristics of the microscopic-monodispersion emulsified fuel by using a Shirasu Porous Glass (SPG) membrane, which was made from volcanic ash called “Shirasu”. The microscopic emulsion from the 1-micrometer pore diameter of the SPG membrane was examined with respect to the combustion characteristics on a direct diesel engine. The major results are as follows; (1) The particle size of gas oil emulsion increases to about 3.5 times the pore size of the original SPG membrane. (2) The particle distribution showed monodispersion and, thus, can make an ideal monodispersion microscopic W/O emulsified fuel for diesel engines. (3) By using the microscopic monodispersion emulsified fuel we can remarkably reduce the both emissions of NOx, CO, HC, Soot and PM, and fuel consumption by 5-15% compared the base gas oil.