A magnetron includes a yoke, an anode unit including an anode cylinder, radially arranged vanes, and first and second pole pieces at both sides of the anode cylinder, a cathode unit having a filament spaced apart from the vanes, and an output unit having an antenna lead connected to one vane to radiate high-frequency microwaves. The first pole piece includes a first flat portion, a slope at an inner side of the first flat portion, a second flat portion at an inner side of the slope and having a diameter of 9.5˜10.5 mm, a first hole formed in the second flat portion and having a diameter of 8˜8.2 mm, and a second hole formed in the slope for penetration of the antenna lead. The magnetron achieves higher and stabilized efficiency, restricted oscillation efficiency variation, lower energy consumption, and improved load stability without deterioration of oscillation efficiency.