A microwave ion source comprising a discharge chamber provided with an ion source seed material inlet and an ion outlet, a means for radiating microwaves in said discharge chamber, a means for applying a magnetic filed to the inside of said discharge chamber, a means for supplying ion source seed material to said discharge chamber through said ion source seed material inlet and an ion extraction electrode, said ion extraction electrode being made of magnetic material having a resistivity of less than 10.sup.6 .OMEGA.cm and a permeability of more than 5. The present microwave ion source has an improved ion current efficiency.