The Fair Trade Commission has searched major department store chains mostly in western Japan on suspicion they formed a price cartel to inflate shipping charges for their products.

The FTC's inspections over the alleged price-fixing practice were conducted Wednesday at seven chains, including Osaka-based Takashimaya Co. and Tokyo-based Daimaru Matsuzakaya Department Stores Co.

The others were Hankyu Hanshin Department Stores Inc., Kintetsu Department Store Co. and Keihan Department Stores Ltd. — all based in Osaka — and Sogo & Seibu Co. and Takashimaya Service Co., both based in Tokyo.

All seven are suspected of breaking the antimonopoly law. All except Takashimaya Service admitted to being searched by the FTC. Takashimaya Co. and Daimaru Matsuzakaya said they will "fully cooperate" with the investigation.

Sources close to the matter said that, for the past several years, the stores are believed to have decided among themselves how much they should raise shipping fees for Japan's traditional midsummer and year-end gift items, among other products.

Around 2015, the companies raised their shipping charges one by one by a margin exceeding ¥10 but not more than ¥100, according to information on their websites and other sources.

The antitrust watchdog believes the companies formed the cartel to cope with increases in shipping caused by parcel delivery companies, the sources said.