To provide a leak preventive cap which can apparently shorten the length of a bottle mouth, less likely to be effected by a design of a container, in which the space between the container mouth and the cap is made tight to prevent a leakage of a content and the bottle mouth is assembled with an elastic cap to display effects to prevent the liquid in the container from flowing out, and which is easy for opening.

The leak preventive cap comprises a movable column part and an elastic part. A movable column part end has a thin column part and a conical shaped block. The elastic part is made of a soft and elastic material, having a through-hole at the center, with its upper part constituting an inclined conical portion gradually expanding upward so as to remain inside the bottle mouth. When the movable column part is inserted into the through-hole of the elastic part by the column part, the elastic part is tightly adhered to the column part of the movable column part and the conical block surface.