Metal base organic transistor (MBOT) is a new type of vertical organic transistor that has simple organic/metal/organic layered structure. MBOT showed a high output current modulation for very low operation voltage, but large off-current resulted in poor ON/OFF ratio around several tens. We found that leaving the device under atmospheric condition caused decrease of the OFF current and the ON/OFF ratio reaching 330 was attained. The effect of exposure to air was investigated from the viewpoint of oxidation layer of electrode surface.