A magnetic field detection device includes a magnetism detection element, a modulator, and a demodulator. The magnetism detection element has a sensitivity axis along a first direction. The modulator is configured to impart a spin torque to the magnetism detection element. The spin torque has a rotational force and oscillates at a first frequency. The rotational force is exerted on a plane including the first direction and a second direction orthogonal to the first direction. The demodulator is configured to demodulate an output signal received from the magnetism detection element and to detect an intensity of a measurement target magnetic field exerted on the magnetism detection element on the basis of an amplitude of the output signal. The output signal has the first frequency.