Secrecy of communications protects both communication content (conversation, email text) and metadata (who called whom, when, duration) from third-party access. Japan's Constitution Article 21(2) states "the secrecy of communications shall not be violated," reinforced by Telecommunications Business Act Article 4.
Protection scope is broad: call content, caller/receiver numbers, call times, durations, email recipients, and web browsing history. Carriers are prohibited from disclosing this information without legitimate reason, with violations carrying up to 3 years imprisonment or 2 million yen fines.
Exceptions include court-warranted law enforcement disclosure, carrier service operations (billing), and user consent. Wiretapping investigations operate under the Communications Interception Act, limited to organized crime with court warrants.
For nuisance call countermeasures, communication secrecy complicates spam filtering - carriers analyzing call content for auto-blocking may violate this right. The Ministry revised guidelines in 2023 to permit certain anti-nuisance measures. See call recording legal guide for details.