A microwave resonator magnetic field measuring device (1) for measuring alternating magnetic fields, with a base plate (11) having at least one supporting/bearing/clamping point (111), at least one mechanical oscillator (12+13) formed as a microwave resonator in the form of a cantilever (13) having at least one magnetostrictive layer (12), the latter being connected and mounted at at least one point to the base plate (11) in the at least one supporting/bearing/clamping point (111), at least one input coupling means (161) for microwaves and at least one output coupling means (162) for microwaves, wherein the base plate (11) and the mechanical oscillator (12+13) formed as a microwave resonator are at least partly electrically conductive and electrically conductively connected to one another. Also, a magnetic field measuring method having a magnetic field measuring device according to the invention.