A hydraulic drive system for a work machine, the system comprising: a first hydraulic actuator; a second hydraulic actuator; a hydraulic pump device configured to supply hydraulic fluid to the first hydraulic actuator and the second hydraulic actuator; a control valve configured to adjust a flow rate of the hydraulic fluid returning from the first hydraulic actuator; a regenerative device configured to supply the hydraulic fluid returning from the first hydraulic actuator to the second hydraulic actuator; a first operation device configured to operate the first hydraulic actuator; a second operation device configured to operate the second hydraulic actuator; and a sensor device including a first operation amount sensor configured to measure an operation amount of the first operation device, a second operation amount sensor configured to measure an operation amount of the second operation device, a first pressure sensor configured to measure a pressure of the bottom side of the first hydraulic actuator, and a second pressure sensor configured to measure a pressure between the hydraulic pump device and the second hydraulic actuator; Characterized in that The hydraulic drive system further comprises a controller including an abnormality detection part configured to determine whether the sensor device is abnormal or not and a first control part (141, 144, 147) that controls the regenerative device such that the hydraulic fluid returning from the first hydraulic actuator is supplied to the second hydraulic actuator when the sensor device is normal and values measured by the sensor device satisfy regenerative conditions that are required to be met when the hydraulic fluid returning from the first hydraulic actuator is supplied to the second hydraulic actuator and such that, when the sensor device is abnormal, the hydraulic fluid returning from the first hydraulic actuator is not supplied to the second hydraulic actuator even if the values measured by the sensor device satisfy the regenerative conditions. a first hydraulic actuator; a second hydraulic actuator; a hydraulic pump device configured to supply hydraulic fluid to the first hydraulic actuator and the second hydraulic actuator; a control valve configured to adjust a flow rate of the hydraulic fluid returning from the first hydraulic actuator; a regenerative device configured to supply the hydraulic fluid returning from the first hydraulic actuator to the second hydraulic actuator; a first operation device configured to operate the first hydraulic actuator; a second operation device configured to operate the second hydraulic actuator; and a sensor device including a first operation amount sensor configured to measure an operation amount of the first operation device, a second operation amount sensor configured to measure an operation amount of the second operation device, a first pressure sensor configured to measure a pressure of the bottom side of the first hydraulic actuator, and a second pressure sensor configured to measure a pressure between the hydraulic pump device and the second hydraulic actuator; Characterized in that The hydraulic drive system further comprises a controller including an abnormality detection part configured to determine whether the sensor device is abnormal or not and a first control part (141, 144, 147) that controls the regenerative device such that the hydraulic fluid returning from the first hydraulic actuator is supplied to the second hydraulic actuator when the sensor device is normal and values measured by the sensor device satisfy regenerative conditions that are required to be met when the hydraulic fluid returning from the first hydraulic actuator is supplied to the second hydraulic actuator and such that, when the sensor device is abnormal, the hydraulic fluid returning from the first hydraulic actuator is not supplied to the second hydraulic actuator even if the values measured by the sensor device satisfy the regenerative conditions.