A magnetron filter apparatus having a bottom box type filter case which is engaged and fixed with a cathode input side of a magnetron bulb and provided with a cover. A transformer which supplies a cathode current to the magnetron is contained in the filter case and a filter is contained in the filter case for preventing leakage of unwanted electromagnetic waves. Further, an insulation box in which the transformer is embedded and which is provided with integrally formed cylindrical portions through which lead wires to a primary winding of the transformer is disposed in the filter case. The insulation box is positioned by insertion of the cylindrical portions thereof into holes formed in a wall of the filter case.