An electronic circuit unit is provided that allows the increase in the size thereof to be suppressed. An electronic circuit unit 1 includes a board, a housing, and a cover. A board mount portion is disposed in the closest position to an outer edge of the housing, except for peripheral walls of the housing. An outer edge of the board is placed on the board mount portion of the housing, and a peripheral portion of the cover is placed on the outer edge of the board. Screws are inserted into screw holes of the housing; spaces defined by recess portions of the board 10 and the peripheral walls; and screw holes of the cover. Then, the outer edge of the board is sandwiched between the board mount portion of the housing and the peripheral portion of the cover.