A small reactor capable of appropriately measuring the temperature of a coil is provided. The reactor includes a coil 2 including a pair of coil elements 2a and 2b and a magnetic core including a pair of inner core portions 31 disposed in the respective coil elements 2a and 2b and outer core portions that connect the inner core portions 31 to form a closed magnetic circuit. Each of the coil elements 2a and 2b has an end face shape having a rounded corner portion 21, which is a corner portion of a rectangle that is rounded. A temperature sensor 7 is disposed in a trapezoidal space between the rounded corner portions 21 of the coil elements 2a and 2b that face each other. The temperature sensor 7 is pressed so as to contact the rounded corner portions 21 by the sensor holder portion 54 provided on an insulator, and is capable of appropriately measuring the temperature of the coil 2. When the temperature sensor 7 is disposed in a region in which the inner core portions 31 are not disposed in the respective coil elements 2a and 2b, the coil elements 2a and 2b can be positioned near each other and the size of the reactor can be reduced.