A substrate unit includes a circuit board; a connector portion mounted on the circuit board; and a case that contains the circuit board. The case includes a lower case having an accommodation section opened upward and accommodating the circuit board. An upper cover covers the accommodation section of the lower case. An aperture is opened in a side wall of the case, allowing the connector portion to be fitted to a counterpart connector portion. The upper cover includes a ceiling portion having a board opposing region that opposes the circuit board accommodated in the accommodation section and an outer region is located outside of the board opposing region without opposing the circuit board; and guide grooves inclined from the board opposing region to the outer region. The guide grooves discharging the water droplets away from the circuit board.