Understanding of metal/organic interfaces is one of the key issues for the design of high performence organic devices. We have studied the interface properties of the Al/pentacene/Au sandwich samples by performing electrostatic force microscopy (EFM) and measuring I-V curves at various temperatures. The Al/pentacene/Au sandwich sample showed a typical rectifying I-V curve as expected from the energy diagram. Interestingly, the current increased with time when a constant forward bias was applied. In order to understand the reason, we performed EFM of the cross section of Al/pentacene/Au to measure the time variation of the charge density across the interface. The result suggested that there was a charge accumulation at the Al/Pentacene interface and that the reason for the current increase with time was the lowered potential barrier caused by the trapped charges. From the temperature-dependent I-V measurement, we could estimate the amount of barrier lowering. When the measurement was performed in UHV environment, the current increase was not observed, which suggested that water molecules diffused into pentacene in the ambient condition played a role of trap sites.