The present invention relates to a cap assembly mounted on a discharging portion of a container for containing an ingredient different from that in accommodated in a container, which comprise a cap body a cap body formed by an upper cap part and a lower cap part which may be divided along a cut-off line; a housing with a storage chamber for a secondary ingredient formed by the lower portion of the upper cap part; a sealing assembly provided below the lower cap part, comprising a sealing element and a space through which the secondary ingredient may be discharged in use; and a mouth formed above the lower cap part. The cap assembly of the present invention has advantageous effects of convenience in use and of preventing alteration thereof.