A cross flow fan comprising: a support plate; and an impeller that is formed by blades disposed in a plurality on the support plate at predetermined intervals, with a lengthwise direction cross-sectional shape of each of the blades having a suction surface arc (Rs) that forms a convex suction surface, a pressure surface arc (Rp) that forms a concave pressure surface, an inner peripheral side arc (Ri) that interconnects a first end of the suction surface arc (Rs) and a first end of the pressure surface arc (Rp), and an outer peripheral side arc (Ro) that interconnects a second end of the suction surface arc (Rs) and a second end of the pressure surface arc (Rp), and with a radius (rp) of the pressure surface arc (Rp) being greater than a radius (rs) of the suction surface arc (Rs), a radius (ri) of the inner peripheral side arc (Ri) being greater than a radius (ro) of the outer peripheral side arc (Ro), wherein the blades are disposed in such a way that the inner peripheral side arcs (Ri) are positioned on an inner peripheral side of the support plate and the outer peripheral side arcs (Ro) are positioned on an outer peripheral side of the support plate, characterized in that: a region of maximum thickness of the blade is located in a position 40% to 60% from the inner peripheral side arc (Ri) in the lengthwise direction; and a flow path width between the plural blades gradually decreases from the inner peripheral side toward the outer peripheral side of the support plate. a support plate; and an impeller that is formed by blades disposed in a plurality on the support plate at predetermined intervals, with a lengthwise direction cross-sectional shape of each of the blades having a suction surface arc (Rs) that forms a convex suction surface, a pressure surface arc (Rp) that forms a concave pressure surface, an inner peripheral side arc (Ri) that interconnects a first end of the suction surface arc (Rs) and a first end of the pressure surface arc (Rp), and an outer peripheral side arc (Ro) that interconnects a second end of the suction surface arc (Rs) and a second end of the pressure surface arc (Rp), and with a radius (rp) of the pressure surface arc (Rp) being greater than a radius (rs) of the suction surface arc (Rs), a radius (ri) of the inner peripheral side arc (Ri) being greater than a radius (ro) of the outer peripheral side arc (Ro), wherein the blades are disposed in such a way that the inner peripheral side arcs (Ri) are positioned on an inner peripheral side of the support plate and the outer peripheral side arcs (Ro) are positioned on an outer peripheral side of the support plate, characterized in that: a region of maximum thickness of the blade is located in a position 40% to 60% from the inner peripheral side arc (Ri) in the lengthwise direction; and a flow path width between the plural blades gradually decreases from the inner peripheral side toward the outer peripheral side of the support plate. a region of maximum thickness of the blade is located in a position 40% to 60% from the inner peripheral side arc (Ri) in the lengthwise direction; and a flow path width between the plural blades gradually decreases from the inner peripheral side toward the outer peripheral side of the support plate.