Abstract: A descent slowing device is disclosed that includes a housing having an input driving shaft and a driving pulley fitted thereon and accommodating therein a gear train and an impeller adapted to be driven through the gear train by a driving pulley about which is trained a rope or line. When one end of the line descends owing to a load acting thereon to rotatively drive the driving pulley and hence the impeller, the rotation of the impeller is restrained by rotation control oil or similar viscous fluid in which the impeller is immersed, thereby controlling the descent of the load at a constant slow speed. According to the invention, the impeller comprises four blades, the rotating speed ratio of the impeller to the input driving shaft being more than 40:1. Moreover, the driving pulley is formed with a rope receiving groove comprising line abutting projections and non-contact recesses alternately arranged.