A detection unit receives an optical signal that has passed through a space to be measured. A spectrum extraction unit extracts a range to be measured from the optical signal received by the detection unit. The spectrum extraction unit extracts an optical signal formed as a gas molecule of a gas to be measured absorbs energy of the optical signal. A determination unit determines the presence of an anomaly in the space to be measured based on a waveform of the optical signal extracted by the spectrum extraction unit.