To provide a water tank playing instrument enabling children to undergo bodily sensation of the status of aquatic organisms while playing through devising to desire that they can experience nature in their playing in view of the fact that children have much feeling of hoping to play.

The water tank playing instrument comprises a water tank 5, a stand 1 supporting the water tank 5, wherein the stand 1 is equipped with a staircase 30 outside, a tunnel 4 for children to have free access to, an observation space 16 reaching the inside of the water tank 5 from midway of the tunnel 4, and a life-support system 7 for aquatic organisms F in the water tank 5. A transparent material is used as an outer cylinder 51 constituting the water tank 5 and as a water tank viewing part 6 constituting the observation space 16 so as to enable children to observe the aquatic organisms F in the water tank 5 both from inside and outside while playing. A stand 3 can be designed to constitute a stationary-step one 3A, variable-step one 3B or split one 2, and the water tank 5 can be placed on the stand 1, that is, can be constituted as an independent-type water tank 5A.