An inner diameter measurement instrument (100) includes a first channel (120) and a second channel (140) respectively communicating with a balloon (110), a fluid injection mechanism (130), a fluid lock mechanism (150), and an amount measurement unit (160). The fluid injection mechanism (130) injects an incompressible fluid into the balloon (110) through the first channel (120) or the second channel (140). The fluid lock mechanism (150) closes the first channel (120) or the second channel (140). The amount measurement unit (160) measures the amount of an incompressible measurement fluid additionally injected after the balloon (110), the first channel (120), and the second channel (140) are filled with the fluid and the fluid lock mechanism (150) closes one of the first channel (120) and the second channel (140), by the fluid injection mechanism (130) through the other of the first channel (120) and the second channel (140).
