To detect vapors from explosives, we have developed a new detection system based on a novel ion source for atmospheric pressure chemical ionization (APCI). The direction of the sample gas flow introduced into the ion source is opposite to that of the ion flow produced by the ion source. We call this technique "counter-flow introduction" (CFI). The CFI ion source has high ionization efficiency for nitro-compounds and is installed in an ion-trap mass spectrometer. Our tests on various military explosives show that this system is suitable for practical on-line explosive detection; namely major explosives can be detected within a few seconds without any pretreatment of the sample gases.