Liquid fossil fuels such as gasoline, diesel oil, and kerosene are widely used as a fuel of various transportation apparatus and generating electricity apparatuses including the automobiles. The spray combustion has been widely used for internal combustion engine to use the fuel efficiently. But some parts of the phenomenon are not elucidated because this combustion method is complicated phenomenon. To elucidate this phenomenon, there are many ways of analyzing droplet. For example, observing a single droplet which suspended by a catenary or under the microgravity. However, those methods are not enough simulation of a real droplet in the internal combustion engine. In this study, we developed an apparatus which could inject a freedom droplet of diameter about 30µm. It is considered that the droplet is in a real internal combustion engine. And the apparatus was installed in a container which could realize elevated temperature and pressure. And a droplet was injected under supercritical condition that simulated the condition in the internal combustion engine and combustion behavior was observed. We experimented with Diesel oil and hexadecane which is a surrogate fuel of Diesel oil. The influences of the combustion area of the droplet, the burning time, and the luminance value at the time of combustion were investigated. As a result, it was found that when the same fuel was burned under the same temperature condition under the supercritical condition, the maximum combustion area increased.