A hydraulic excavator is equipped with one or more downsizing scheme including at least one of: (i) a bubble removing device for removing bubbles in hydraulic fluid, (ii) a hydraulic tank including a main tank and a variable capacity tank arranged separately from each other, and (iii) a variable capacity tank or hydraulic tank which is fitted to the rear surface of the upper cover of the seat-side block. Thus, the lateral section of the seat can be ultimately downsized to provide a large space around the seat. Then, unlike the prior art, a canopy or a cab of a larger hydraulic excavator can be securely arranged on the upper swing body of a small hydraulic excavator. Since such large parts can be commonly used with large hydraulic excavators, it is possible to significantly reduce the cost of the hydraulic excavator.