is spot welded at 2 to 4 positions e g 3 positions excluding welding portions between the lower portion 26 a of the thick plate 26 and a bent section 52 which will be described later approximately l shaped hook members 36 and 38 extend externally from the upper sides of the thick plates 26 and 28 openings 40 and 42 having the number same as that of the hook members 36 are formed in the side surface which extend in the vehicle width direction on one side right side of fig1 and 4 of the casing 20 whereas openings 44 46 having the number same as that of the hook members 38 are formed in the side surface which extends in the vehicle width direction on the other side left side in fig1 and 4 the openings 40 42 are disposed so that the upper sides thereof have the same height in the row of the openings 40 and 42 the openings 42 are disposed on both ends of the row and the openings 40 are disposed between the openings 42 the length of the opening 42 in the up and down directions is greater than that of the opening 40 in the up and down directions the hook members 36 of the thick plate 26 project externally or outwardly of the casing 20 through the openings 40 and 42 the openings 44 46 are disposed so that the upper sides thereof have the same height in the row of the openings 44 and 46 the openings 44 are disposed between the openings 46 the length of the opening 46 in the.