A medicinal device includes a first member; and a second member configured to be attachable to and detachable from the first member, the second member being a part of a medicinal-liquid administering device and including: a first member comprising a first-member-side power transmission part, and a second member comprising a second-member-side power transmission part; wherein the first-member-side power transmission part and the second-member-side power transmission part are configured to be engaged with each other for administering medicinal liquid into a living body; and wherein the second member is configured to open a housing covering the first-member-side power transmission part to engage the second-member-side power transmission part with the first-member-side power transmission part at a time of attaching the first member to the second member.