vertical wall 36 a extend in parallel with a peripheral wall 38 of the lower cover 14 and is coupled to a vertical wall 36 b that extends along one edge of the starter cut out portion 34 b a vertical wall 36 c that extends along the other edge of the starter cut out portion 34 b extend in parallel with the vertical wall 36 b and is coupled to the peripheral wall 38 of the lower cover 14 protruding end surfaces of the vertical walls 36 a to 36 c are disposed at the same level in height as a protruding end surface of the peripheral wall 38 of the lower cover 14 and shaft portions of the battery connecting bolt 28 a and starter connecting bolt 28 b protrude upward by a large amount the other bolt receiving section 24 b is provided near the alternator connecting bolt 28 c with an alternator cut out portion 34 c whose protruding side end edge portion is cut out the other bolt receiving section 24 b is also provided with a vertical wall 40 a that extends along one edge of the alternator cut out portion 34 c and with a vertical wall 40 b that extends along the other edge of the alternator cut out portion 34 c and along the vertical wall 40 a the vertical wall 40 a is coupled to the peripheral wall 38 of the lower cover 14 but the vertical wall 40 b is not coupled to the peripheral wall 38 of the lower cover 14 a clearance having a size sufficient to receive a fitting wall 44 discussed below is defined between the vertical wall 40 b and the peripheral wall 38 of the lower cover 14 protruding end surfaces of.