Chiba Prefectural Police used the wiretapping law in November to listen in on a group suspected of using parcel delivery services to traffic drugs.
The Tokyo, Kanagawa and Aichi police departments are the only other prefectural forces to have used the law for specific criminal investigations since it took effect in August 2000, according to the National Police Agency.
Chiba police obtained a wiretapping warrant from the Chiba District Court to intercept mobile phone communications from a suspected central figure of the ring, sources said.