PROBLEM TO BE SOLVED: To provide a planter in which an existing planter is used as it is and the capability of a swollen water-absorbing resin can be fully exerted, and to provide a container for a water retention agent which is used therefor.SOLUTION: A container for a water retention agent, which is held in a planter for growing a plant, is closed with a lid having a plurality of holes through which the roots of the plant can pass, and a swollen water-absorbing resin is put therein. In the planter, there is soil on the upper side of the lid of the container for a water retention agent, and the container for a water retention agent is filled with a gel made of a swollen water-absorbing resin which has been swollen with water.
