To improve speed and maintain operability of a work machine, while recovering/reproducing potential energy with the work machine.

A potential energy recovering/reproducing device comprises a work machine 6 having a boom 13 with the base end being pivotably secured to a rotary frame 30, boom cylinders 16, 16 connected to the rotary frame 30 and the boom 13, an assist cylinder 18 connected to the rotary frame 30 and the boom 13, and an accumulator 85 accumulating press oil delivered from the assist cylinder 18 during lower rotation operation of the boom 13. The assist cylinder 18 is arranged so that the axis of the assist cylinder 18 is arranged in a recessing position with respect to the axis of the boom cylinders 16, 16 during the upper rotation operation of the boom 13.