A seismic sensor comprising a measurement unit configured to measure acceleration; an earthquake determination unit configured to perform an earthquake determination processing determining whether or not an earthquake has occurred based on the acceleration measured in a predetermined determination period; and an index calculator configured to calculate an index value indicating a scale of an earthquake in an earthquake processing period after the predetermined determination period, when the earthquake determination unit determines that an earthquake has occurred, where in the earthquake processing period, when an index value calculated by the index calculator is equal to or larger than a predetermined threshold, a shut off determination unit outputs a shut-off signal for shutting off an operation, to an external device or a related device provided together, the seismic sensor further including: a continuous earthquake determination unit configured to determine again in the earthquake processing period, whether or not the earthquake has occurred, based on the acceleration measured in the earthquake processing period; characterized in that the shut-off determination unit configured to inhibit output of the shut-off signal regardless of the index value when the continuous earthquake determination unit determines that no earthquake has occurred; and when the continuous earthquake determination unit detects a pulse impact waveform in the earthquake processing period, a shut-off signal is not output and, the process returns to a standby state or to the earthquake determination processing. a measurement unit configured to measure acceleration; an earthquake determination unit configured to perform an earthquake determination processing determining whether or not an earthquake has occurred based on the acceleration measured in a predetermined determination period; and an index calculator configured to calculate an index value indicating a scale of an earthquake in an earthquake processing period after the predetermined determination period, when the earthquake determination unit determines that an earthquake has occurred, where in the earthquake processing period, when an index value calculated by the index calculator is equal to or larger than a predetermined threshold, a shut off determination unit outputs a shut-off signal for shutting off an operation, to an external device or a related device provided together, the seismic sensor further including: a continuous earthquake determination unit configured to determine again in the earthquake processing period, whether or not the earthquake has occurred, based on the acceleration measured in the earthquake processing period; characterized in that the shut-off determination unit configured to inhibit output of the shut-off signal regardless of the index value when the continuous earthquake determination unit determines that no earthquake has occurred; and when the continuous earthquake determination unit detects a pulse impact waveform in the earthquake processing period, a shut-off signal is not output and, the process returns to a standby state or to the earthquake determination processing. a continuous earthquake determination unit configured to determine again in the earthquake processing period, whether or not the earthquake has occurred, based on the acceleration measured in the earthquake processing period; characterized in that the shut-off determination unit configured to inhibit output of the shut-off signal regardless of the index value when the continuous earthquake determination unit determines that no earthquake has occurred; and when the continuous earthquake determination unit detects a pulse impact waveform in the earthquake processing period, a shut-off signal is not output and, the process returns to a standby state or to the earthquake determination processing.