An object of the present invention is to provide a device for preventing a sealing member from jumping out of a syringe of an injection container, which can easily and reliably prevent the sealing member from sliding out by an injection gas pressure. A pop-out prevention device includes a syringe holder portion that can be coaxially mounted on a syringe, and a cap portion that can be coaxially mounted on the syringe and that can be fixed to the syringe holder portion with a finger flange of the syringe interposed therebetween. And the cap portion includes a contact portion 23c that contacts the seal member 13 and prevents the seal member from moving in the axial direction. [Selection diagram] FIG.【課題】注射容器のシリンジ内を密封的に摺動可能なシール部材が注入ガス圧力により外部へ飛び出すことを簡単かつ確実に防止できる、シール部材の飛び出し防止具を提供する。【解決手段】飛び出し防止具は、シリンジ１１に同軸に装着可能なシリンジホルダ部２２と、シリンジに同軸に装着可能であり、シリンジのフィンガーフランジ１１ｃを挟んでシリンジホルダ部に固着可能なキャップ部２３とを備え、キャップ部は、シール部材１３に当接しシール部材の軸方向の移動を阻止する当接部２３ｃを備える。【選択図】図４
