To provide a low temperature fluid boosting pump system for preventing an overspeed caused in a hydraulic motor of a driving source by sudden reduction in a load caused by a gas inflow, in a low temperature fluid boosting pump driven by the hydraulic motor.

The low temperature fluid boosting pump system includes the low temperature fluid boosting pump 20 driven by the hydraulic motor 32 for boosting a low pressure-low temperature fluid introduced from a low temperature fluid tank, an opening adjustable control valve 33 arranged in a hydraulic flow passage 31 for connecting the hydraulic motor 32 and a hydraulic pump 30 for supplying hydraulic pressure to the hydraulic motor 32, and a control part 40 for adjusting an opening of the control valve 33 in response to reduction in a flow rate value of the low temperature fluid measured at the upstream side of the low temperature fluid boosting pump 20.