PURPOSE: To accurately detect an oil state by providing a 1st oscillation circuit which outputs variation frequency signal corresponding to the output variation of an oil state sensor and a 2nd oscillation circuit which outputs a reference frequency signal in the same device.

CONSTITUTION: A device base part is hollowed and the 1st and 2nd oscillation circuits 20 and 30 and an oil state detecting circuit are put in the hollow part while provided on the same substrate. The circuit 20 oscillates at a frequency f1 corresponding to variation in the electrostatic capacity CS of an oil deterioration sensor 10, a capacitor 23 is charged with a specific time constant through a timer 21 and then discharged when its voltage reaches a threshold voltage, thereby outputting a variation frequency signal i1. The circuit 300 ocelots at the same frequency f2 with a signal i1 in a new oil state, and a capacitor 34 is charged with a specific time constant through a timer 31 and then discharged when its voltage reaches a threshold voltage, thereby outputting a variation frequency signal i2. Then the circuit 40 compares the frequencies f1 and f2 that the signals i1 and i2 have with each other and outputs an oil deterioration signal i3. A display circuit 50 makes a display 51 of the deterioration degree of il corresponding to the signal i3.