To measure a dangerous fluid in safety by providing a plurality of temperature measuring resistor element means having resistance variable in response to heat transfer of a fluid flowing on the heater elements, a heating circuit for the heaters, a sensor circuit, a plurality of overheat prevention circuits and the like.

The current detector comprises a power supply input circuit 30, a heating circuit 40 for heater and a temperature measuring resistor amplifying circuit 50. The heating circuit 40 for heater is provided with a fixed resistor R3 connected between the series joint of a heater element 4 and a fixed resistor R1 and the input terminal of an operational amplifier U1. Since the fixed resistor R3 suppresses the current flowing into the heater element 4 through the operational amplifier U1, the current flowing into the heater element 4 can be suppressed even when an undue DC voltage is applied to the heater element 4 due to failure of the operational amplifier U1. According to the circuitry, heating temperature of the heater element 4 can be prevented from exceeding over the firing temperature of inflammable mixture gas by selecting the resistance of the fixed resistor R3 appropriately.