In the obstacle-detection device proposed, an emitter (8) transmits an HF interrogation signal (11) across the zone under surveillance to a surface-wave identification mark (1; a, b, c, ...) which has an antenna (5) to receive the interrogation signal (11) and emit a response signal (12a, 12b, ...) and an inter-digital transducer (3;, 3a, 3b, ...) designed to convert the HF interrogation signal (11) into surface waves and to convert surface waves reflected by a reflector into response signals (12a, 12b, ...). A receiver/processing unit (9) receives the response signals (12a, 12b, ...) and processes them in accordance with processing specifications. The invention is intended for use in monitoring railway level crossings.