すみません、ご飯のおかわりをいただけますか？ — Excuse me, could I have a refill of rice? (Cultural · Dining, Polite to Staff, JLPT N5)

Gesture & etiquette

Catch the eye of the staff; hold up your rice bowl slightly if needed to indicate what you're asking for; a brief, appreciative nod when the refill arrives is standard.

おかわり' (okawari = refill/seconds) is a normal and welcome request at Japanese restaurants, especially for rice — many traditional set-meal restaurants (定食屋) offer free rice refills; asking clearly and politely is never considered impolite or gluttonous.