To increase the degree of freedom of a hydraulic piping and an electric wiring route arranged in a space by a method wherein the space between a under cover and a lower frame is sufficiently ensured.

An under cover 15 comprising a central bent part 15a bent in a mountain-form manner such that a central part in the direction of the width of a car is protruded upward; and two end bent parts 15b bent downward such that a part of two ends in the direction of a car width is protruded further upward than the central bent curve part 15a and in a state to be fixed at a car body frame 5 is arranged between the lower panel 9 of the car body frame 5, at which a battery box 7 is fixed, and the battery box 7. A support plate 23 arranged on the lower panel 9 to support an under cover 15 is arranged between the central bent part 15a and the two end bent parts 15b and closer to the two end bent parts 15b. A drain hole 9a through which sewage dripping from a battery box 7 and flowing over the under cover 15 is discharged to the outside of the car body frame 5 is formed in the lower panel 9 situated further outside in the direction of a car width than the support plate 23.