Effects of a slit injection (SI) on exhaust noise from a rectangular hypersonic nozzle were investigated experimentally. In the Japan Aerospace Exploration Agency (JAXA), the pre-cooled turbojet (PCTJ) engine specified for a hypersonic transport is under development. In this study, a 1/100-scaled model of the rectangular hypersonic nozzle for the PCTJ is manufactured. The main jet (MJ) is injected to the atmosphere through the nozzle. The total pressure of the MJ was set at the takeoff condition of the vehicle. From slit shaped converging nozzles mounted at the end of the ramp, air is injected to the MJ. The sound pressure level (SPL) around the nozzle was measured with microphones. Using the SI, especially in the ramp direction, the SPL was decreased by 1 to 2dB at wide frequencies up to 50kHz. At the same time, the Overall SPL (OASPL) decreased monotonously with the increase in the momentum ratio of the SI and the MJ. It is considered that the governing parameter of the SI is the momentum ratio. The growth rate of the shear layer was estimated from Pitot pressure distribution. It is implied that the shear layer growth rate is linked to the acoustic radiation.