A cap assembly having a storage chamber for a secondary ingredient, which is adapted to a mouth of a container, comprising: a body having a mouth; a chamber part having a storage space for secondary ingredient in the body and a hole formed at the lower end thereof; said chamber part is sealed by a movable working member and is opened when the movable working member is removed from the hole upon removing the cap so that the secondary ingredient may be mixed with the first ingredient in the container, said the movable working member being adapted to maintain the opened hole, after moving to open the hole, thereby effectively mixing of the different ingredients.