To provide a seal structure in which a needle and a seal do not slide and which is easily mountable and removable, and also to provide a device provided with the seal structure. Provided are a seal, aseal structure which is provided with the seal, and a device which is provided with the seal structure. The seal allows the needle of a valve device to be passed therethrough, the valve device comprising a fluid chamber which has an outflow hole and an inflow hole, and also comprising the needle having a front end reciprocating within the fluid chamber. The seal comprises: a body section consisting of an elastic body and having formed therein a needle insertion hole through which the needle is passed; and a flange section consisting of an elastic body and extended annularly and radially outward from the body section. The seal is characterized in that the diameter (D3) of the opening at one end of the needle insertion hole is smaller than the diameter (D2) of the opening at the other end of the needle insertion hole.