Inner diameter measuring device for measuring the internal diameter of the conduit (100), the second flow path member (120) first flow passage member which is in communication individually for (110) the balloon member (140), a fluid injection mechanism (130 (150), and a (160) capacity measuring means outflow sealing mechanism,). Fluid injection mechanism (130), injecting an incompressible fluid inside the balloon member (110) through (140) the second flow path member (120) or the first flow path member. Outflow sealing mechanism (150), to close one second flow path member (120) or first flow path member (140). Capacitance measuring means (160), fluid is filled in (140) the second flow path member (110), the first flow path member balloon member (120), outflow sealing mechanism (150) the first flow path member (120) or from the closed state while the second flow path member (140), and measuring the capacitance to be measured an incompressible fluid to be injected more fluid injection mechanism (130) through the other.導管の内径を測定する内径測定装置(100)は、バルーン部材(110)に対して個別に連通している第一流路部材(120)および第二流路部材(140)、流体注入機構(130)、流出密閉機構(150)、および容量測定手段(160)を有する。流体注入機構(130)は、第一流路部材(120)または第二流路部材(140)を通じてバルーン部材(110)の内部に非圧縮性の流体を注入する。流出密閉機構(150)は、第一流路部材(120)または第二流路部材(140)の一方を閉止する。容量測定手段(160)は、バルーン部材(110)、第一流路部材(120)および第二流路部材(140)に流体が充填され、流出密閉機構(150)により第一流路部材(120)または第二流路部材(140)の一方を閉止した状態から、他方を通じて流体注入機構(130)によりさらに注入される非圧縮性の測定流体の容量を測定する。
