External force acting on a hydraulic excavator bucket is calculated precisely. An external force measurement system for a work machine includes a hydraulic cylinder for driving a front unit of the work machine and a plurality of strain gauges for sensing strain amounts of a cylinder rod of the hydraulic cylinder. The plurality of strain gauges are formed of at least two sets of strain gauges. The at least two sets of strain gauges are arranged to oppose each other as viewed in an axial direction of the cylinder rod. The external force measurement system includes a load calculation section that is configured to calculate a load acting on the hydraulic cylinder based on difference between the strain amounts of the strain gauges arranged to oppose each other in each set.