Abstract: The ultrasonic wave sensor (2) according to the invention comes into contact with a wall at a high temperature (1) by means of a wave guide (5) made of a vitro-ceramic material. The sensor includes, inside a body (3) made of metallic material, a piezoelectric ceramic pellet (8) connected to an electrical power supply or measurement leads (9). The wave guide (5), also placed in the body (3), is in contact by one of its ends (5b) with the piezoelectric pellet (8) and by its other end (5a) with the wall (1). The body (3) of the sensor is mounted in an articulated manner on a support (21) fixed to the wall (1). The invention applies in particular to the measurement of flow rate in a primary duct of a pressurized-water nuclear reactor.