A new type of leak detector (LeakD) was developed using the principle of selective and sensitive oxygen detection using oxygen-ion-conducting ceramics, e.g., yttria-stabilized zirconia (YSZ). By comparing the experimental results of the LeakD with a quadrupole mass spectrometer (QMS) and other methods, it was clear that (1) at a large leak, the data of LeakD coincided with theoretical values; (2) at a small leak, the P(O-2)(vac) calculated from the electromotive force (emf) of LeakD differed from the P(O-2)(vac) of QMS, especially at higher temperature for YSZ; (3) at a small leak, using tandem two LeaksDs under both sensing surfaces of YSZ evacuated with any different oxygen partial pressures about 10(0)-10(-6),P(O-2)(vac) obtained from the emf of LeakD nearly coincided with the theoretical values; and (4) a leak point is found by blowing nitrogen gas on the point. The phenomena of (2) and (3) showed that the oxygen-ion-conducting ceramics may be especially influenced by a total pressure less than 10(-3) or 10(-4) Torr. The simple and small LeakD has good reproducibility and rapid response, and it is assumed that it has a smaller outgassing rate compared to some gauges with filaments. The LeakD can continuously detect a leak in the routine installation, and also inform others of an accident or emergency. The LeakD is a convenient and highly sensitive leak detector compared to a traditional helium leak detector using a QMS.