An earth terminal mounting structure includes a boss protruding from a conductive case; and an earth terminal fixed to a distal end surface of the boss. The earth terminal is provided with a terminal-side stopper that extends along a side surface of the boss. A boss-side stopper having a protrusion shape is provided on the side surface of the boss. The boss-side stopper faces the terminal-side stopper in a circumferential direction of the boss. The terminal-side stopper and the boss-side stopper restrict rotation of the earth terminal in a clockwise direction and in a counter-clockwise direction.