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This paper reports experimental results for a long-term field trial of 1-GHz clock differential phase shift quantum key distribution that was incorporated in the test-bed optical fiber network installed in the Tokyo metropolitan area. A photon transmitter and a photon receiver were connected to a 90-km loop-back optical link with a 30-dB loss. Superconducting single photon detectors were employed to detect photons with a low dark count rate. Stable maintenance-free operation was achieved for 25 days, where the average sifted key generation rate and the quantum bit error rate were 11 kb/s and 2.6%, respectively. The average generation rate of a secure key after error correction and privacy amplification was 1.1 kb/s with a fluctuation of ± 0.5 kb/s. The achieved performance was examined using detailed bit error analysis. A comparison with meteorological data revealed the way in which the field environment influenced the key distribution performance.