It is made possible to prevent damage to hydraulic parts of a work machine due to a decrease in performance of a hydraulic fluid even when the hydraulic fluid of a hydraulic system is changed and the hydraulic fluid becomes a mixed oil in which a mineral oil and a biodegradable oil are mixed with each other. For this purpose, a storage device stores, in advance, oil change history information of the hydraulic fluid, oil kind determination values for determining whether the hydraulic fluid is either the mineral oil or the biodegradable oil or the mixed oil of the mineral oil and the biodegradable oil, and abnormality determination values for respective kinds of the hydraulic fluid. A controller checks whether the oil change history information includes oil change information for which oil kind determination has not been made. When there is oil change information for which oil kind determination has not been made, the controller determines a kind of the hydraulic fluid on the basis of the respective values of a plurality of oil properties detected by an oil sensor and the oil kind determination values. The controller determines an abnormality in the hydraulic fluid by using abnormality determination values corresponding to a result of determination of the kind of the hydraulic fluid.