An inner cap attachment-detachment structure for capping a container, comprising a plug-like inner cap and an outer cap mounted to the inner cap so as to be externally overlaid thereupon, wherein the outer cap can be used as a hand tool to combine and attach/detach the inner cap to/from the container in the manner of a bayonet engagement, wherein the outer cap is equipped, on the upper area, with an attachment-detachment operation part which has a circular peripheral shape in a plane view and is capable of being embedded in a recessed part of the inner cap; furthermore, on an outer periphery of the attachment-detachment operation part, an engagement recessed groove is formed in a plurality, and the engagement recessed groove comprises a receiving notch for leading the embedding to the recessed part of the inner cap, and a catch groove which is connected to the receiving notch and extends in a peripheral direction; furthermore, on the catch groove, a raised part is arranged to stabilize an engagement between the outer cap and the inner cap when the engagement between the two is realized, a lifting rib is formed in the upper area which is divided by the catch groove, and the lifting rib sets a lower surface side thereof as a lifting surface; on the other hand, the inner cap comprises: a plug body which is adapted to be embedded in an aperture part of the container in the manner of a plug and closes the aperture part, and a recessed part which is formed on the upper inner side of the plug body and receives the attachment-detachment operation part of the outer cap; furthermore, above the plug body, an engagement projection is formed in a plurality to project toward the inner side of the recessed part in a plane view; characterized in that a pressing surface, which is a lower surface of the catch groove in the attachment-detachment operation part, is formed such that a diametrical dimension of the pressing surface is larger than that of the lifting surface, so as to be capable of pressing the plug body from above while in a state of the engagement with the inner cap. the outer cap is equipped, on the upper area, with an attachment-detachment operation part which has a circular peripheral shape in a plane view and is capable of being embedded in a recessed part of the inner cap; furthermore, on an outer periphery of the attachment-detachment operation part, an engagement recessed groove is formed in a plurality, and the engagement recessed groove comprises a receiving notch for leading the embedding to the recessed part of the inner cap, and a catch groove which is connected to the receiving notch and extends in a peripheral direction; furthermore, on the catch groove, a raised part is arranged to stabilize an engagement between the outer cap and the inner cap when the engagement between the two is realized, a lifting rib is formed in the upper area which is divided by the catch groove, and the lifting rib sets a lower surface side thereof as a lifting surface; on the other hand, the inner cap comprises: a plug body which is adapted to be embedded in an aperture part of the container in the manner of a plug and closes the aperture part, and a recessed part which is formed on the upper inner side of the plug body and receives the attachment-detachment operation part of the outer cap; furthermore, above the plug body, an engagement projection is formed in a plurality to project toward the inner side of the recessed part in a plane view; characterized in that a pressing surface, which is a lower surface of the catch groove in the attachment-detachment operation part, is formed such that a diametrical dimension of the pressing surface is larger than that of the lifting surface, so as to be capable of pressing the plug body from above while in a state of the engagement with the inner cap.