To provide a water retaining bag for water supply to plants, which enables water supply to plants without a fear of leak.

This water retaining bag 1 for water supply to plants includes a bag part 2 and a leak preventing part 3. The bag part 2 has an opened upper end as a plant inserting opening 5, and a lower part as a water storage part 6 which can store water. The leak preventing part 3 is attached to the plant inserting opening 5, and extended to the inside of the bag part 2 from the plant inserting opening 5 to make the inside thereof serve as a plant inserting passage to insert plants with roots or cut roots. The leak preventing part 3 is extended to a length which does not allow to reach the water storage part 6, and the side thereof is set with a space 4 between the leak preventing part 3 and the inside of the bag part 2, and a volume of the space 4 is more than that of the water storage part 6.