A sensor control circuit unit (1) includes a circuit board (70) which has opposite sides (70a) and (70b) facing in parallel with each other, and carries a signal detection circuit (28) for receiving a detection signal whose level is equal to or less than one μA and which changes in accordance with NOX concentration, a power supply circuit (35), and a heater circuit (40) for controlling a heater. The power supply circuit (35) and the heater circuit (40) are mounted on one side of the circuit board (70) toward one side (70a) of the opposite sides, and the signal detection circuit (28) is mounted in a mounting area on the other side of the circuit board (70) toward the other side (70b) of the opposite sides, the mounting area being different from a mounting area of the circuits (35) and (40). A ground conductor (24x) connected to the ground is continuously formed on a surface of the circuit board (70) on which the signal detection circuit (28) is mounted such that the ground conductor (24x) extends along at least the other of the opposite sides.