Detection device, detection system, and propagating member

A detection device, comprising:

• a detector transmitting an ultrasonic wave and detecting a reflected wave;
• a first propagating member mounted to the detector, the ultrasonic wave propagating through the first propagating member;
• a second propagating member, the second propagating member being softer than the first propagating member, the ultrasonic wave propagating through the second propagating member after propagating through the first propagating member;
• and
• a fixture including a pressing part that presses a periphery of the second propagating member toward the first propagating member,
• the first propagating member including
• a first surface crossing a first direction, the first direction being from the first propagating member toward the second propagating member, and
• a third surface oblique to the first surface,
• the second propagating member contacting the first and third surfaces,
• the second propagating member including
• a first region pressed by the pressing part, and
• a second region surrounded with the first region,
• a distance between the first surface and the pressing part in the first direction being less than a thickness of the first region in the first direction.