A signal acquisition unit acquires a plurality of signal groups acquired based on backscattered light from a plurality of long gauge length sections and a plurality of signal groups acquired based on backscattered light from a plurality of short gauge length sections, which are set in an optical fiber used for distributed acoustic sensing. A signal selection unit selects a long gauge length section based on a first signal feature of each of signal groups of the plurality of long gauge length sections, and selects a plurality of short gauge length sections corresponding to the selected long gauge length section. A section estimation unit determines one short gauge length section, as a section in which an event has occurred, based on a second signal feature of each of signal groups of the plurality of selected short gauge length sections.