A teeth strengthening agent is provided to be safely used for an oral composition or a food, promote re-calcification of de-calcified tooth enamel materials and improve acid-resistance of tooth enamel and tooth dentinal materials. A teeth strengthening agent comprises seaweed mineral and acid as effective ingredients. The agent further comprises xylitol. An oral composition comprises the seaweed mineral containing teeth strengthening agent, wherein the seaweed mineral is 0.1-20 wt.%. A food comprises the seaweed mineral containing teeth strengthening agent, wherein the seaweed mineral is 0.1-6 wt.%.
