Surface potential properties at the interface of pentacene thin films on gold (Au) and aluminum (Al) surfaces were investigated by using a near-field scanning microwave microprobe (NSMM). The surface potential formed across the pentacene film was observed by measuring the microwave reflection coefficient $S_{11}$ and compared with the result of a Kelvin-probe method. The obtained reflection coefficient ${Delta}S_{11}$ of the pentacene thin films on Al was decreased as the pentacene film thickness increased due to the increased accumulation of negative space charges, while for Au ${Delta}S_{11}$ was essentially constant.