An air blast type slag cut-off device for steel converter, adapted for jetting compressed air or gas from a nozzle toward the tap hole of the steel converter to force the slag coming out of the tap hole back into the steel converter. The device has a shut-off valve disposed at a portion of the high-pressure gas pipe just upstream from and in the close proximity of the nozzle. The high-pressure gas pipe and an operating fluid supply pipe for operating the shut-off valve are connected to respective sources through a common rotary joint or through independent flexible hoses, so that the steel converter can be tilted smoothly while maintaining the fluid communication between the pipes and respective sources. The time lag of jetting of the air or gas from the nozzle is minimized thanks to the reduced length of the passage between the shut-off valve and the nozzle.