Provided is a filter for a pet water supply device with which adequate filtration performance can be reliably obtained. The present invention comprises: a water storage tank 1 for a pet water supply device, the water storage tank 1 being configured such that water drinking parts 31 are provided above the water storage tank 1, and the water supplied from the water storage tank 1 to the water drinking parts 31 returns to the water storage tank 1; and a filter 5 for the pet water supply device, the filter 5 being detachably provided between the water drinking parts 31, and being configured so that the water returning from the water drinking parts 31 to the water storage tank 1 is caused to pass from the upper-surface side to the lower-surface side of the filter 5. The present invention also comprises: a filter tray 6 in which an outer circumferential wall 62 is provided rising from a bottom wall 61 in which water passage holes 64 are formed; an interior filtration material 71 accommodated inside the filter tray
