The flow rate measuring device 1 includes a pipe 4 and a sensor section 2 detachable from the pipe 4. The tube 4 is provided with a transparent tube wall 41 capable of visually recognizing the blood taken outside the body from the outside and a light shielding film 30 covering a part of the tube wall 41. The light-emitting element opening 31 and the light-receiving element opening 32 are formed in the light-shielding film 30. The sensor section 2 includes a light emitting element 10 that emits laser light, a light receiving element 20 that receives laser light, and a belt 71 that detachably fixes the sensor section 2 to the tube 4. The light emitting element opening 31 is smaller than the light emitting surface 11 of the light emitting element 10. The light receiving element opening 32 is smaller than the light receiving surface 21 of the light receiving element 20. When the sensor section 2 is fixed to the tube 4 by the belt 71, the light emitting element opening 31 is included in a part of the light emitting surface 11 and the light receiving element opening 32 is included in a part of the light receiving surface 21. A part of the laser light emitted by the light emitting element 10 passes through the opening 31 for the light emitting element and enters the inside of the tube wall 41. Measurement light that is a laser beam that has been reflected by the blood in the tube wall 41 and has passed through the aperture 32 for the light receiving element and has undergone the Doppler effect due to the flow velocity is incident on the light receiving element 20.流速計測装置1は、管4と、管4に脱着可能なセンサ部2を備えている。管4は、体外に取り出した血液を外部から視認できる透明な管壁41と、管壁41の一部を覆う遮光膜30を備えている。遮光膜30に発光素子用開口31と受光素子用開口32が形成されている。センサ部2は、レーザー光を発光する発光素子10と、レーザー光を受光する受光素子20と、センサ部2を管4に脱着可能に固定するベルト71を備えている。発光素子用開口31が発光素子10の発光面11より小さい。受光素子用開口32が受光素子20の受光面21より小さい。ベルト71によって管4にセンサ部2を固定すると、発光面11の一部に発光素子用開口31が含まれ、受光面21の一部に受光素子用開口32が含まれる。発光素子10が発光したレーザー光の一部が発光素子用開口31を通過して管壁41内に入射する。管壁41内の血液で反射して受光素子用開口32を通過したレーザー光であって流速によるドップラー効果を受
