The Osaka District Court ruled Friday that information collected by police via GPS devices without a warrant cannot be accepted as evidence because it is an illegal invasion of privacy.
The decision contradicts a court ruling in January, suggesting the courts are divided over the issue.
The Criminal Procedure Law has no provision that directly refers to GPS operations.
Friday's ruling concerned a case in which the Osaka Prefectural Police installed GPS devices in 19 cars for about six months to investigate a theft suspect and his acquaintance, without first obtaining a warrant.
The district court determined that the investigation "significantly violated privacy" rights. It also said the police "were likely to have been issued a warrant by the court if they had requested," but that they "did not even consider whether they should obtain one."
The suspect's lawyer said this may have been the first court decision in the nation to declare such investigations illegal.
Prosecutors insist that GPS terminals are tools to "aid" investigators when they tail suspects or conduct a stakeout. They also described the violation of privacy as "low" because investigators can "only roughly determine the location information and do not record (the data) 24 hours (a day)."
But presiding Judge Takaaki Nagase rejected the prosecutors' arguments, saying that investigations using GPS devices are "highly accurate" and are "different in nature" from activities that rely solely on the human eye.
"The accuracy is likely to improve as technology develops. There should be a need to consider ways to check future illegal investigations," he said.
The theft suspect is a 43-year-old man from Osaka Prefecture who was arrested in December 2013 for allegedly stealing cars and other items in the Kansai region between February 2012 and September 2013.