A diagnostic system for a work machine includes a data storage device (210) in which sensor information (A) input from a sensor (101A) that senses an oil property of oil used for operation of the work machine and an abnormality degree determination value defined for each kind of the sensor information (A) are stored, and an arithmetic processing device (104) that executes first processing of discriminating the degree of abnormality level of the oil based on the sensor information (A) and the abnormality degree determination value, second processing of determining whether or not necessity to carry out oil analysis involving oil extraction exists based on the degree of abnormality level of the oil discriminated in the first processing, and third processing of outputting information indicating that the oil analysis is necessary to other terminal if it is determined that the oil analysis is necessary in the second processing.