A plastic ampule of the present invention is composed of a single layer of a plastic layer consisting only of a cyclic polyolefin with a glass transition temperature of 50° C. to 104° C.； integrally includes an ampule body formed in a bottle shape with a top part and a bottom part and having a bung for discharging a drug solution on the top part, a cap attached to the ampule body so as to seal the bung, and a thinner part having a layer thickness thinner than those of the ampule body and the cap, formed along a circumferential direction of the bung and connecting between the ampule body and the cap； and is opened by breaking off or twisting off the thinner part as a cutting part between the ampule body and the cap.
