A multiblade fan (1, 101, 201, 301, 401) comprising: an impeller (10, 410) including a rotating plate (12, 412) secured to a rotary shaft and a plurality of blades extending from a surface of the rotating plate (12, 412) and circumferentially spaced about the rotary shaft; a fan casing (20, 420) containing the impeller (10, 410), the fan casing (20, 420) having a circumferential wall facing a periphery of the impeller (10, 410) and a first end face disposed adjacent to distal ends of the plurality of blades, the circumferential wall extending at an increasing distance from the rotary shaft in a rotation direction of the impeller (10, 410), the first end face having an inlet (22, 422) through which air flows into the fan casing (20, 420); a duct connected to a downstream end of the fan casing (20, 420) in an air flow direction, the duct having an outlet through which the air from the fan casing (20, 420) is discharged; and wherein the duct includes a diffuser plate extending from an upstream end of the circumferential wall in the air flow direction and the diffuser plate extends in the rotation direction outwardly in a radial direction of the impeller (10, 410), wherein the circumferential wall includes a tongue that is bent part of the upstream end and the tongue is connected to the diffuser plate, wherein the first end face has a bell mouth (23, 423) provided at the inlet (22, 422) and the bell mouth (23, 423) protrudes into the fan casing (20, 420), characterized by a flow regulating block (40, 140, 240, 340, 440) disposed on an inner surface of the first end face and regulating a flow of the air, wherein the flow regulating block (40, 140, 240, 340, 440) is spaced from the circumferential wall and extends along the bell mouth (23, 423) in the rotation direction such that the flow regulating block (40, 140, 240, 340, 440) is located between 0 and 120 degrees in the rotation direction from a reference position on a line connecting the rotary shaft to a tip of the tongue. an impeller (10, 410) including a rotating plate (12, 412) secured to a rotary shaft and a plurality of blades extending from a surface of the rotating plate (12, 412) and circumferentially spaced about the rotary shaft; a fan casing (20, 420) containing the impeller (10, 410), the fan casing (20, 420) having a circumferential wall facing a periphery of the impeller (10, 410) and a first end face disposed adjacent to distal ends of the plurality of blades, the circumferential wall extending at an increasing distance from the rotary shaft in a rotation direction of the impeller (10, 410), the first end face having an inlet (22, 422) through which air flows into the fan casing (20, 420); a duct connected to a downstream end of the fan casing (20, 420) in an air flow direction, the duct having an outlet through which the air from the fan casing (20, 420) is discharged; and wherein the duct includes a diffuser plate extending from an upstream end of the circumferential wall in the air flow direction and the diffuser plate extends in the rotation direction outwardly in a radial direction of the impeller (10, 410), wherein the circumferential wall includes a tongue that is bent part of the upstream end and the tongue is connected to the diffuser plate, wherein the first end face has a bell mouth (23, 423) provided at the inlet (22, 422) and the bell mouth (23, 423) protrudes into the fan casing (20, 420), characterized by a flow regulating block (40, 140, 240, 340, 440) disposed on an inner surface of the first end face and regulating a flow of the air, wherein the flow regulating block (40, 140, 240, 340, 440) is spaced from the circumferential wall and extends along the bell mouth (23, 423) in the rotation direction such that the flow regulating block (40, 140, 240, 340, 440) is located between 0 and 120 degrees in the rotation direction from a reference position on a line connecting the rotary shaft to a tip of the tongue.