In the event that an abnormality is detected in a throttle valve driving apparatus, an upper limit value for a throttle valve opening is set according to a vehicle speed when a brake switch is not switched on. The invention allows for the passage of ammonia through microporous hydrophobic gas-permeable membranes and its capture in a circulated stripping solution with concomitant production of a concentrated non-volatile ammonium salt. The guitar formatting circuit formats the digital string signals to be compatible with a digital communication protocol, and outputs the formatted digital string signals.