A rotation number detector according to an embodiment is a rotation number detector that detects the number of rotations of a magnet attached to a rotating body by using a power generation unit. The power generation unit includes N (N is a natural number equal to or larger than 1) power generation elements, each including a magnetic wire in which magnetization reversal occurs due to a large Barkhausen effect and a pickup coil that is wound around the magnetic wire. The magnetic wire is longer than a wound portion of the pickup coil in an extension direction of the magnetic wire of the power generation elements. The magnetic wire is set above a rotation center of the magnet.