A reactor unit includes reactors; and a cooler. The reactors are disposed in at least one line on a reactor cooling surface that is one of outer surfaces of the cooler. The cooler has a cooling medium flow passage that is in contact with an inner surface on a reverse side of the reactor cooling surface. The cooling medium flows linearly from an inlet portion to an outlet portion of the cooling medium flow passage. A direction in which the cooling medium flows inside the cooling medium flow passage is same as a direction in which the reactors are disposed in the at least one line. Cooling fins are provided on the inner surface on the reverse side of the reactor cooling surface. A longitudinal direction of each cooling fin is same as the direction in which the cooling medium flows inside the cooling medium flow passage.