Abstract: An improved ultrasonic ranging device having a ultrasonic sensor unit provided for detecting the distance between a ground object and a rear end of a vehicle includes: a casing body composed of a front casing and a rear casing with a round opening formed in the opposing ends; a control unit with a plurality of microswitches and a control circuit installed at one end of the casing body and electrically connected to the ultrasonic sensor unit; a gear system with a motor and a reduction gear member disposed at another end of the casing body and electrically connected to the control circuit; and a protective cover with a transmission arrangement movably connected to the casing body with the transmission arrangement meshed with the reduction gear and the control unit; thereby, the protective cover can be automatically raised up and lowered down along with the backward and forward movements of a vehicle for protecting the ultrasonic ranging device against water, dust and mud.