A fluidized bed boiler comprising - a furnace limited by walls, a grate and a roof, as well as - a steering funnel arranged in connection with the furnace below the grate for leading bed material from the furnace downwards, the steering funnel for bed material in a fluidized bed boiler comprising - a mainly downwards tapering horizontal cross-section, - an upper edge comprising an inlet connection for leading bed material from a furnace inside the steering funnel, - a lower edge comprising an outlet connection for removing bed material from the steering funnel, - a path for bed material arranged between the inlet and outlet connections, whereby inside the steering funnel between its upper and lower edge are arranged guide plates (14, 15, 19) which are substantially divergent from a surface of the steering funnel for guiding a flow of bed material inside the steering funnel, characterized in that guide plates (14, 15, 19) are arranged inside the steering funnel as a rectangular inner funnel which has inclined side walls, thus the funnel being mainly downwards tapering on its horizontal cross-section to permit a flow of bed material on substantially whole cross-sectional area of the steering funnel whereby - the upper edge of the inner funnel comprises an inlet opening for material and the lower edge comprises an outlet opening, - the inlet opening of the inner funnel is arranged approximately to the same height as the inlet opening of the steering funnel, - the outlet opening of the inner funnel opens approximately to the middle of the height of the steering funnel, a little above the midpoint, - the inner funnel is arranged to throttle a material flow at the centre of the steering funnel, i.e. at the outlet opening, thus creating a condition for easier flowing of material between the inner funnel and the steering funnel, - to the outer edges of the inner funnel are attached extra guide plates, arranged to throttle the flow of material outside the inner funnel from angles of the walls towards the centre of the walls. - a furnace limited by walls, a grate and a roof, as well as - a steering funnel arranged in connection with the furnace below the grate for leading bed material from the furnace downwards, the steering funnel for bed material in a fluidized bed boiler comprising - a mainly downwards tapering horizontal cross-section, - an upper edge comprising an inlet connection for leading bed material from a furnace inside the steering funnel, - a lower edge comprising an outlet connection for removing bed material from the steering funnel, - a path for bed material arranged between the inlet and outlet connections, whereby inside the steering funnel between its upper and lower edge are arranged guide plates (14, 15, 19) which are substantially divergent from a surface of the steering funnel for guiding a flow of bed material inside the steering funnel, characterized in that guide plates (14, 15, 19) are arranged inside the steering funnel as a rectangular inner funnel which has inclined side walls, thus the funnel being mainly downwards tapering on its horizontal cross-section to permit a flow of bed material on substantially whole cross-sectional area of the steering funnel whereby - a mainly downwards tapering horizontal cross-section, - an upper edge comprising an inlet connection for leading bed material from a furnace inside the steering funnel, - a lower edge comprising an outlet connection for removing bed material from the steering funnel, - a path for bed material arranged between the inlet and outlet connections, - the upper edge of the inner funnel comprises an inlet opening for material and the lower edge comprises an outlet opening, - the inlet opening of the inner funnel is arranged approximately to the same height as the inlet opening of the steering funnel, - the outlet opening of the inner funnel opens approximately to the middle of the height of the steering funnel, a little above the midpoint, - the inner funnel is arranged to throttle a material flow at the centre of the steering funnel, i.e. at the outlet opening, thus creating a condition for easier flowing of material between the inner funnel and the steering funnel, - to the outer edges of the inner funnel are attached extra guide plates, arranged to throttle the flow of material outside the inner funnel from angles of the walls towards the centre of the walls.