To shift to an electronic instrument by adding a simple circuit by A/D converting a voltage signal corresponding to the temperature of a heating element, obtaining an oil pressure corresponding to heating temperature using a temperature-oil pressure conversion table and displaying it.

A fixed resistance R1 which is connected to a direct current power source V2 at one end and to a connection point 31 at the other end and a thermistor 27 which is connected to the connection point 31 at one end and provided with the other end are mutually connected in series to compose a temperature detecting circuit 29. The thermister 27 is arranged adjacently to a metal bar 23 and resistance value varies according to the heating temperature of a nichrome wire 21. The heating temperature is equivalent to the heating temperature of a sender side nichrome wire 15 is series, and resistance value of the thermister 27 varies according to the heating temperature of the nichrome wire 15. The temperature detecting circuit 29 outputs a voltage signal corresponding to the heating temperature through the connection point 31. A digital processing block 5 converts the voltage signal through an A/D converting circuit 33, and an oil pressure computing section 35 obtains an oil pressure using a temperature-oil pressure conversion table and displays it on a display section 39.