A vial assembly comprising (a) a vial having a lip, the lip having an annular outer circumferential wall; (b) a plug that seals up an opening of the vial; (c) a plastic-made cap for the plug that seals up the opening of the vial, said cap comprising: a top wall portion having an outer circumference and centrally defining an opening, and a cylindrical skirt portion extending downwardly from said outer circumference of said top wall portion and being open at a lower end thereof, and said skirt portion having an inner diameter (a) greater than an outer diameter (b) of said lip of said vial, characterized in that said skirt portion is provided on an inner wall thereof with at least two elastically deformable ribs formed at an angular interval in a direction perpendicular to said top wall portion such that, upon application of said cap onto said lip with said opening of said vial being sealed with said plug, said ribs can be elastically deformed to facilitate the application of said cap onto said lip and further to prevent separation of said cap from said lip, whereby said ribs are located opposite an outer circumference of said lip of said vial. (a) a vial having a lip, the lip having an annular outer circumferential wall; (b) a plug that seals up an opening of the vial; (c) a plastic-made cap for the plug that seals up the opening of the vial, said cap comprising: a top wall portion having an outer circumference and centrally defining an opening, and a cylindrical skirt portion extending downwardly from said outer circumference of said top wall portion and being open at a lower end thereof, and said skirt portion having an inner diameter (a) greater than an outer diameter (b) of said lip of said vial, characterized in that said skirt portion is provided on an inner wall thereof with at least two elastically deformable ribs formed at an angular interval in a direction perpendicular to said top wall portion such that, upon application of said cap onto said lip with said opening of said vial being sealed with said plug, said ribs can be elastically deformed to facilitate the application of said cap onto said lip and further to prevent separation of said cap from said lip, whereby said ribs are located opposite an outer circumference of said lip of said vial. a top wall portion having an outer circumference and centrally defining an opening, and a cylindrical skirt portion extending downwardly from said outer circumference of said top wall portion and being open at a lower end thereof, and said skirt portion having an inner diameter (a) greater than an outer diameter (b) of said lip of said vial, characterized in that said skirt portion is provided on an inner wall thereof with at least two elastically deformable ribs formed at an angular interval in a direction perpendicular to said top wall portion such that, upon application of said cap onto said lip with said opening of said vial being sealed with said plug, said ribs can be elastically deformed to facilitate the application of said cap onto said lip and further to prevent separation of said cap from said lip, whereby said ribs are located opposite an outer circumference of said lip of said vial.