According to one embodiment, a sensor includes an element part, and a control circuit part. The element part includes first and second elements. Each of the first and second elements includes a first magnetic element and a first conductive member. The control circuit part includes a first current circuit, a differential circuit, and a phase detection circuit. The first current circuit is configured to supply a first current to the first conductive member. The differential circuit is configured to output a differential signal corresponding to a difference of a first signal and a second signal. The first signal corresponds to a change in a first electrical resistance of the first magnetic element of the first element. The second signal corresponds to a change in a second electrical resistance of the first magnetic element of the second element. The phase detection circuit is configured to perform a phase detection of the differential signal.