The drug solution administration device (10) includes a reuse part (14) and a disposable part (12) that can be attached to the reuse part (14). The cover part (92) provided in the opening (100) of the housing (90) that houses the reuse side power transmission part (88) that can be engaged with the disposable side power transmission part (24) is provided in the reuse part (14). When the disposable part (12) is attached to the contact part (12), the contact part (132) of the disposable part (12) is contacted, opened against the urging force of the urging member (94) and maintained in the open state. When the disposable part (12) is removed from (14), the opening (100) of the housing (90) is closed by the biasing force of the biasing member (94).薬液投与装置（１０）は、リユース部（１４）と、リユース部（１４）に装着可能なディスポ部（１２）とを備える。ディスポ側動力伝達部（２４）に係合可能なリユース側動力伝達部（８８）を収容する筐体（９０）の開口部（１００）に設けられたカバー部（９２）は、リユース部（１４）へのディスポ部（１２）の装着時にディスポ部（１２）の当接部（１３２）に接触し、付勢部材（９４）の付勢力に抗して開くとともに開状態に維持され、リユース部（１４）からディスポ部（１２）が取り外された際に付勢部材（９４）の付勢力により筐体（９０）の開口部（１００）を閉塞するように閉じる。
