It’s a water-ripple pattern.

The water-ripple pattern can be often combined with other motifs such as flowers (cherry blossoms or reeds), or small animals (butterflies, goldfish or water birds).

It is one of the traditional patterns which first appeared on a clay pot or bronze ware from the Yayoi era.

After the early-modern times, ryusui-mon has evolved from the common pattern to high-class pattern filled with grace and dignity by employing the formal elements of art.