A hydraulic excavator drive system includes: a cylinder that drives a moving part that is an arm or a bucket. Hydraulic oil is supplied from a hydraulic pump to the cylinder via a control valve. A bypass line branches off from a rod-side supply/discharge line. The bypass line is blocked and opened by a restrictor. The restrictor is controlled by a controller such that, when the hydraulic oil is supplied to the cylinder through a head-side supply/discharge line, the restrictor blocks the bypass line if a pressure detected by a load detector is lower than a predetermined value, and opens the bypass line if the pressure detected by the load detector is higher than or equal to the predetermined value.