In a magnetron for microwave oven, a high-voltage input terminal structure 30 is provided on a shield box 26 mounted on a magnetron unit 22. The high-voltage input terminal structure 30 has a cylindrical earth electrode 33, a cylindrical high potential inner electrode 32 and a central conductor 31a. The cylindrical earth electrode 33 is secured to the shield box 26 and the cylindrical high potential inner electrode 32 is coaxially arranged in the cylindrical earth electrode 33. The central conductor 31a is connected through an inductor 28 to a cathode lead 27 of the magnetron unit 22. An insulating resin 34b is filled in a gap between the cylindrical earth electrode 33 and the highpotential electrode 32 to form a capacitor section C. Cylindrical insulating sheaths 37, 38 are formed by the resin on the both sides of the capacitor section C. The capacitor section C and the cylindrical insulating sheaths, 37, 38 are integrally formed and continuously extended along a straight line.