A reactor that can measure the temperature of the magnetic core is provided. The reactor includes an assembly having a magnetic core and a coil that has a winding portion, and a temperature sensor that measures the temperature of the reactor. The magnetic core has an inner core portion that is inserted into the winding portion, and a sensor disposition groove is formed in an outer peripheral face of the inner core portion and is disposed inside the winding portion. The temperature sensor is provided inside the sensor disposition groove. According to this configuration, it is possible to precisely measure the temperature of the magnetic core while the reactor is operating.