The fire started in the Fuji Seoul restaurant on Merrick Avenue in Merrick just after midnight.
Flames spread to several other businesses.
About 125 firefighters from Merrick, Wantagh, Roosevelt and Freeport battled the blaze for about two hours.
The firefighters battled heavy flames through the roof, and homes in the vicinity were evacuated as a precaution.
Five businesses were damaged.
One firefighter was taken to Nassau University Medical Center after suffering exhaustion.
He was evaluated and later released.
Detectives have determined the fire was not intentionally set, but the cause remains under investigation.
Merrick Avenue was closed at Miller Place as fire department activity continued.
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