Disclosed is a riser top structure in which a lateral duct (15) for providing a connection between a riser (1) and a cyclone separator (2) comprises an introduction part (16) at the connection with the riser (1), said introduction part (16) having the same cross-sectional area as the riser (1), and a constriction part (17) between the introduction part (16) and the cyclone separator (2), the cross-sectional area of the constriction part (17) steadily decreasing from the introduction part (16) towards the cyclone separator (2) so as to be able to increase the flow speed of combustion gas.