An oil diagnosis system including a controller configured to diagnose a machine on a basis of sensor data about a viscosity, a density, and a dielectric constant of oil, one or more oil property sensors configured to provide sensor data to the controller via a storage device, the controller configured to process the sensor data to obtain, for each of the viscosity, density, and dielectric constant of the oil, three amount-of-change index values, which are a moving average obtained by sequentially determining time-series average values, an amount of change per unit time, and a difference of an amount of change between values at different sensing points in time, the controller comprising: a storage section configured to store abnormality determination values for respective amount-of-change index values, the abnormality determination values each being specified for a corresponding one of the viscosity, density, and dielectric constant of the oil; and an abnormality determining section configured to determine abnormality of the oil on a basis of sensor data of the viscosity, density, and dielectric constant of the oil, and the abnormality determination values stored in the storage device in association with the amount-of-change index values; the controller further comprising: a cause identifying section configured to: calculate said amount-of-change index values; and identify a cause of the abnormality of the oil by checking a combination of the determined changes in each of the viscosity, the density, and the dielectric constant against correlations between causes of abnormality of oil and variation in each of the viscosity, the density, and the dielectric constant and on the basis of the type of the oil determined to be abnormal; and a transmission section configured to transmit, to an external terminal (111, 112), a result for the cause of the abnormality identified by the cause identifying section, wherein the amount-of change index values used for identifying the cause of abnormality of the oil by the cause identifying section are set for each causes of abnormality of the oil. a storage section configured to store abnormality determination values for respective amount-of-change index values, the abnormality determination values each being specified for a corresponding one of the viscosity, density, and dielectric constant of the oil; and an abnormality determining section configured to determine abnormality of the oil on a basis of sensor data of the viscosity, density, and dielectric constant of the oil, and the abnormality determination values stored in the storage device in association with the amount-of-change index values; the controller further comprising: a cause identifying section configured to: calculate said amount-of-change index values; and identify a cause of the abnormality of the oil by checking a combination of the determined changes in each of the viscosity, the density, and the dielectric constant against correlations between causes of abnormality of oil and variation in each of the viscosity, the density, and the dielectric constant and on the basis of the type of the oil determined to be abnormal; and a transmission section configured to transmit, to an external terminal (111, 112), a result for the cause of the abnormality identified by the cause identifying section, wherein the amount-of change index values used for identifying the cause of abnormality of the oil by the cause identifying section are set for each causes of abnormality of the oil. calculate said amount-of-change index values; and identify a cause of the abnormality of the oil by checking a combination of the determined changes in each of the viscosity, the density, and the dielectric constant against correlations between causes of abnormality of oil and variation in each of the viscosity, the density, and the dielectric constant and on the basis of the type of the oil determined to be abnormal; and a transmission section configured to transmit, to an external terminal (111, 112), a result for the cause of the abnormality identified by the cause identifying section, wherein the amount-of change index values used for identifying the cause of abnormality of the oil by the cause identifying section are set for each causes of abnormality of the oil.