An apparatus includes a hermetically sealed housing formed of conductive material. Determination section 101 determines whether or not a mobile terminal is present in a cell that wireless base station 10a manages. Diagnostic circuitry diagnoses operation of the process control loop which may include field devices of the process control loop based upon the digitally modulated analog signal. The oblique segments connect between the axially inner and outer circumferential segments, whereby the zigzag circumferential groove extends in the form of a trapezoidal wave.