A method and a system for automatically calibrating a sound level meter which achieves accurate and secure calibration of a sound level meter using a sound calibrator. The sound calibrator comprises a code signal generator and a calibration sound pressure generator. The sound level meter comprises a code signal discriminator, a mode switch for switching the mode into a calibration mode in response to the discrimination of the code signal discriminator, a gain adjuster for adjusting a gain of an amplifier such that the indicated value corresponds to the level of the calibration sound pressure in the calibration mode, and a calibration recorder for recording the calibration date and time, the sound pressure, the type, and the manufacturing number of the sound calibrator.