The companies in question were visited without notice.
All four companies stated that they are in full cooperation with the authorities on the issue.
JFTC checks steelmakers over cartel suspicion
Japan's Fair Trade Commission (JFTC) has reportedly inspected three major Japanese steel producers - Nippon Steel Corp., JFE Holding Corp. and Sumitomo Metal Industry Ltd - on suspicion that the steel producers in question along with the construction equipment company Kubota Corp. have formed a cartel to fix domestic prices for some steel products used in construction.