In a mounting structure of an electrical junction box, the electrical junction box is vertically mounted on a wall surface along a vertical direction in a state where mating faces of an under case member and an upper case member are set in the vertical direction. Each of the under case member and the upper case member has a pair of left and right locking projections provided therein. A bracket including a backside plate part and a pair of case hook parts provided upright from both sides of the backside plate part is disposed along the vertical direction on the wall surface. The electrical junction box is slidingly inserted from above along the backside plate part in the vertical direction until both pairs of locking projections of the under case member and the upper case member are locked to the pair of case hook parts. Thus, the electrical junction box is mounted on the wall surface by use of the bracket.