A formaldehyde detector (10) includes a detection reagent that produces a color by reaction with formaldehyde. The detector (10) further includes a detection element (30) including: a base material (31) constituting an optical waveguide through which light propagates while repeating total reflection; and a mesoporous layer (32) that is formed on at least part of a light-reflecting surface constituting a surface of the base material (31) and carries the detection reagent. The detector (10) further includes: a light emitter for emitting light to the base material (31); and a light receiver for receiving light emitted from the base material (31). In addition, the detector (10) includes a signal processing part. The signal processing part receives an optical signal from the light receiver, detects a change in optical property due to color production resulting from a reaction between formaldehyde and the detection reagent, and based on the change in optical property, detects the presence of formaldehyde and concurrently determines the concentration thereof.
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