Provided is a technique that can suppress erroneous determination of noise as an earthquake in a seismic sensor and erroneous output of a shut-off signal. The seismic sensor includes: an earthquake determination unit configured to determine an occurrence of an earthquake based on acceleration measured in a determination period after shifting from a power saving mode to a measurement mode with higher power consumption when measured acceleration exceeds a predetermined threshold; and an index calculator configured to calculate an index value indicating a scale of an earthquake in an earthquake processing period after the determination period, when the earthquake determination unit determines that an earthquake has occurred. The seismic sensor causes output of a shut-off signal when the index value is equal to or larger than a threshold in the earthquake processing period. The seismic sensor further includes: a continuous earthquake determination unit configured to determine whether or not an earthquake has occurred, based on acceleration measured in the earthquake processing period; and a 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.