We report a Nobeyama Radioheliograph (NoRH) microwave observation of propagating features of nonthermal emission in a solar flare on 1999 August 28, an M2.8 event for the GOES soft X-ray class. The flare had a very extended source (~4.5×104 km) that was well resolved by the NoRH, and it is confirmed to be a magnetic loop by comparison with the photospheric magnetic field. In the rising phase of the microwave burst, a nonthermal gyrosynchrotron source was observed by the high-rate (10 images per second) observation of the NoRH to propagate from one end of the loop to the other with a speed of 6×103 km s-1. We also found a 9×104 km s-1 propagation in the same apparent position, which is the first imaging observation of streaming electrons.