Flush-wall, single-port and twin-port injections into supersonic flow were investigated experimentally to detail the formation process of a streamwise vortex. A suction-type, supersonic wind tunnel was used. Unheated atmospheric air was ingested into a vacuum tank through a two-dimensional contoured nozzle and a test section. The jet injection angle was 15 degrees. The mainstream Mach number was 2.4. Velocity distributions were measured with Particle Image Velocimetry (PIV). The Mie scattering method was also used. The results revealed that the incoming mainstream flow from behind the jet formed a streamwise vortex near the injection port. This vorticity pair was caused by the difference between jet and mainstream velocity. Its high elliptical vorticity regions rotated to the downstream; then, these regions existed parallel with the mainstream. When the distance between the centers of the twin-port injection holes was 2.6 times the diameter of the injection hole, twin-port injection reduced these vortex widths to the downstream because of the interaction of the inside vortex.