An infrared spectrophotometer, which is capable of preventing measurement data measured from being adversely affected by moisture vapor. An internal space of an attachment receiving therein an optical element is sealed and isolated from ambient air by providing a tube and others, and a dehumidifier is provided in the internal space of the attachment for an optical path. Thus, the dehumidifier is disposed in a relatively narrow space, so that the internal space of the attachment can be efficiently dehumidified within a short period of time so as to reduce humidity in the internal space to suppress absorption of moisture vapor which would otherwise occur on the optical path in the internal space of the attachment. This makes it possible to prevent a negative influence of moisture vapor on measurement data, while reducing a standby time due to the dehumidification.