An elastic cap detachably mountable to a barrel tip portion of a barrel so as to seal a tip of a needle hole includes a cylindrical receiving hole configured to receive at least part of a barrel tip portion and a puncture needle of a barrel. The receiving hole includes a first inner diameter portion having a smaller diameter than an outer diameter of the annular head； a stopper portion provided at an end of the first inner diameter portion； and a second inner diameter portion that is formed from the stopper portion to a bottom wall provided at an end of the second inner diameter portion, the second inner diameter portion having a smaller diameter than the first inner diameter portion.
