To effectively prevent a fire grate from being damaged by overheat by respectively providing a water-cooling duct on the total surface sections of a fixed fire grate row and a movable fire grate row.

Fire grates comprise fixed fire grates 2a and movable fire grates 2b, and the fixed fire grates 2a and the movable fire grates 2b are alternately provided into a staircase form, and fire grate rows are formed by arranging a large number of rows in the lateral horizontal direction, and in addition, a stoker is formed by arranging a large number of fire grate rows in the longitudinal sliding direction. The fire grates 2a and 2b are a rectangular thick plate- form member, and the front and the rear bend into an L-shape downward, and a U-shape string groove 8 which is fitted on the side surface of a fire grate supporting rod 7, is provided at the rear end section. Also, on the front surface section, a feeding port 4 and a discharging port 5 of cooling water are provided on the one end side fire grate of the fire grate row, and a folded back section is provided on the other end side fire grate, and a water-cooling duct for a water cooling means which forms a U-shape duct, is provided.