Even in a location where, for example, a notch section is formed, sufficient rigidity is secured. For this purpose, in reinforcing structure of a front case, a horizontal hemming-bent section 18 formed by folding work is disposed not only in a notch section 9, but also in regions near neighborhood regions 15 and 16. As a result, in the neighborhood regions 15 and 16 of the notch section 9, there is formed a location in which the horizontal hemming-bent section 18 overlaps with a sidewall section 7 along a side end edge to thereby provide reinforcement. Hence, also in the notch section 9, sufficient rigidity is secured.