A rubber stopper 1 for a drug container is used for a drug container 2 having a main body and a neck portion having a diameter smaller than the main body. The rubber stopper 1 includes a main body 3, and a container insertion portion 4 extending downward from the main body 3 and capable of fitting into an opening portion of the container 2. The container insertion portion includes an outer surface portion 40 which comes into contact with an inner surface of the neck portion of the container when attached to the container 2, an upper inner portion which forms a columnar gap portion 45 extending in a substantially uniform shape downward from a central portion of a lower surface of the main body 3, and inclined surfaces 61 and 62 extending obliquely downward toward the outer surface portion 40 of the container insertion portion 4 from the lower end of the upper inner surface portion. The upper end portion 48 of the outer surface portion annularly and liquid-tightly comes into contact with the inner surface of the neck portion of the container, and the lower ends of the inclined surfaces reach or approach the outer surface portion of the container insertion portion.
