Problem to be solved: to obtain a high measurement accuracy by securing the airtightness of the connection portion by connecting the connection member to the connecting member in a flat shape as it is, while using a balloon which is easy to form.The balloon 5 is formed in a bottomed cylindrical shape and has its proximal end opened. The connection member 10 has a flat cylindrical connector portion 12 inserted into the proximal end of the balloon 5. The long diameter of the connector portion 12 is set larger than the maximum outer diameter of the tip end of the balloon 5.Diagram【課題】成形が容易なバルーンを用いながら、狙い通りの扁平形状にして接続部材と接続し、その接続部分の気密性を確保して高い測定精度が得られるようにする。【解決手段】バルーン５は、有底筒状に形成されて基端部が開口している。接続部材１０は、バルーン５の基端部に差し込まれる扁平筒状のコネクタ部１２を有している。コネクタ部１２の長径がバルーン５における先端側の最大外径よりも大きく設定されている。【選択図】図５
