[Problem] To provide a plastic cap with an IC tag without interrupting transmission and reception of information to and from the IC tag, reliably inputting product information to the IC tag and reliably outputting product information that is input.[Means for Solution] The plastic cap has an IC tag 10 storing product information provided on the upper surface of a top plate 21. The top plate 21 is formed by an annular flange 35 and a recessed portion 37, the recessed portion 37 including a side wall 37a continuous to the inner circumferential edge of the annular flange 35 and a bottom 37b continuous to the lower end of the side wall 37a. The IC tag 10 is attached to the annular flange 35 so as to cover the recessed portion 37. A gap between the IC tag 10 and the bottom 37b is maintained to such a degree that the transmission and reception of signals to and from the IC tag 10 are not interrupted when the back surface of the bottom 37b comes in contact with water.