A water tank cleaner capable of cleaning corners and therearound of the inner wall surface of a water tank. The water tank cleaner is provided with a cleaner body 2 mounted in a water tank, and also with an outer operating member 5 mounted outside the water tank and capable of magnetically attracting the cleaner body 2. The cleaner body 2 is provided with a left and right pair of rocking bodies 3 which is adapted to be rockable between a parallel state in which the pair of rocking bodies 3 is arranged parallel to each other in the left-right direction and a bent state in which one of the rocking bodies 3 is bent to the rear side of the other rocking body 3. When mounted on a flat surface of the inner wall surface of the water tank, the cleaner body 2 is set in the parallel state. When mounted to the corner and therearound between adjacent flat surfaces of the inner wall surface of the water tank, the cleaner body 2 is set to the bent state.
