A magnetron for microwave oven has an anode cylinder, vanes (height: H (mm)), a cathode spirally extending along the central axis, a pair of end hats (outer diameter: DEH1 (mm), DEH2 (mm)) fixed to both ends of the cathode, and a pair of pole pieces. Vanes extend from an inner surface of the anode cylinder to the central axis. Free ends of the vanes form a vane inscribing circle (diameter: Da (mm)). Pole pieces expand like funnels from through-holes (inner diameter: DPP1 (mm), DPP2 (mm)) facing to the end hats and pinch the cathode. The shape of the magnetron satisfies, H≦8.5, H/Da≦0.95, DEH1/DPP1≦0.8, DEH1/DPP2≦0.8, DEH2/DPP1≦0.8, DEH2/DPP2≦0.8, 0.92≦Da/DPP1≦0.95, and 0.92≦Da/DPP2≦0.95.