An observation system includes: a power supply unit having a battery; a sensor unit which detects a state of a structure on the basis of electricity from the power supply unit; a charging unit which charges the battery from renewable energy; and a processing unit which processes sensing information in one observation mode of a plurality of observation modes including at least a first observation mode and a second observation mode in which at least one of a sensor capability of the sensor unit and a load of computational processing using the sensing information is higher than in the first observation mode. The processing unit sets an observation mode on the basis of information for disaster occurrence estimation.