Provided is a diagnosis technique with high reliability when a state of lubricating oil of a rotary machine such as a wind power generator is diagnosed by a state monitoring sensor. A diagnosis system of lubricating oil includes a lubricating oil utilization device, a lubricating oil tank for storing lubricating oil to be supplied to the lubricating oil utilization device, and a sensor which measures characteristics of the lubricating oil. In this system, the state of the lubricating oil is diagnosed by using sensor data obtained after a time required for air bubbles in the lubricating oil generated at the time of use of the device disappear elapses since the lubricating oil utilization device is regularly or irregularly stopped.