PURPOSE: To fire a solid fuel in a short time by spraying a liquid fuel such as petroleum on a solid fuel such as wood, coal, and refuse in a combustor wherein the solid fuel and the liquid fuel are both used for combustion.

CONSTITUTION: A combustor is adapted such that a fuel spraying nozzle 7 is mounted on a side wall of a solid fuel combustion unit 3 provided on an upper part of a fire grate 2 in a combustion cylinder 1 toward an introduced solid fuel, and a changeover valve 8 is mounted on a fuel supply pipe 5 between a fuel supply pump 6 and a a liquid fuel combustion burner 4, and further the changeover valve 8 and the fuel spraying nozzle 7 are connected. The changeover valve 8 opens the side of the fuel spraying nozzle 7 prior to combustion of the solid fuel to spray the fuel from the fuel spraying nozzle 7 to the solid fuel over a predetermined time, and thereafter changes over the changeover valve 8 to fire the liquid fuel combustion burner 4 for blowing out a flame and hence firing the solid fuel in a short time.