A medical container including a gasket. Before liquid is injected and discharged (first state), sufficient air-tightness and liquid-tightness are maintained by strong force of fitting of an elastic ring member, fitted on a large diameter part of a gasket body member, relative to a circular tube body. When liquid is injected and discharged (second state), which is a state in which the elastic ring member has been slid from the large diameter part of the gasket body member to a position where the elastic ring member is fitted on the small diameter part of the gasket body member, the fitting force of the elastic ring member, fitted on the small diameter part of the gasket body member, relative to the circular tube body is smaller than the fitting force before liquid is injected and discharged (first state). The construction reduces sliding resistance of the gasket against the circular tube body.