To provide a lower part structure of a cargo bed of a truck capable of simplifying the structure and reducing the cost.

A longitudinal floor joist 5 has a substantially U-shaped section comprising a side plate 6 erected along the vertical direction, a lower plate 7 extending from a lower end of the side plate 6 inwardly in the vehicle width direction, and an upper plate 8 extending from an upper end of the side plate 6 inwardly in the vehicle width direction. The width W2 of the lower plate 7 which is the distance in the vehicle width direction from an outer surface of the side plate 6 and a distal end of the lower plate 7 is set in a range of 25% and 60% of the width W1 along the vehicle width direction of an upper surface of a side frame 1, the lower plate 7 being placed on the upper surface of the side frame.