To diagnose a change with time of a limiting current type oxygen sensor.

A first resistor 38 is connected between an inverting input terminal and an output terminal of a first amplifier 36, and a first oxygen sensor 37 is connected to the inverting input terminal, so that a cell voltage can be impressed between the first amplifier and the first oxygen sensor 37. A second resistor 48 is connected between an inverting input terminal and an output terminal of a second amplifier 46 and a second oxygen sensor 37 is connected to the inverting input terminal, so that the cell voltage can be impressed between the second amplifier and the second oxygen sensor 37. Moreover, a gain of the second oxygen sensor is adjusted beforehand so that a voltage at the output terminal becomes equal to a voltage of the first amplifier 36 under the same oxygen concentration. For example, in diagnosing the first oxygen sensor 37, the cell voltage based on the voltage at the output terminal of the second amplifier 46 is impressed to the first oxygen sensor 37 and outputs of the first and second amplifiers 36 and 46 are compared with each other.