A gas analyzer comprising: a probe member comprising an attachment portion attachable to a flow path wall of a flow path through which an analyte gas flows in a state in which a part thereof is inserted through an opening provided in the flow path wall, an analytical member having a second connection portion detachably attached to a first connection portion located at a base end opposite to a direction of the probe member being inserted into the flow path and located outside of the flow path when attachment portion is connected to the flow path wall and a calibration member that has a third connection portion detachably attached to the second connection portion of the analytical member, a calibration reflection portion, and a calibration area defined therein for introducing the calibration gas, wherein the probe member has a reflective portion and a measurement area defined therein for introducing the analyte gas; the analytical member has a light emission portion and a light reception portion; in a state in which the analytical member is mounted to the probe member, the light emission portion irradiates measurement light toward the measurement area, a reflection portion reflects the measurement light incident on the measurement area, and the light reception portion receives the measurement light reflected by the reflection portion; the probe member has a window portion isolating the measurement area from outside of the base end side and transmitting the measurement light; in a state in which the calibration member is mounted to the analytical member, the light emission portion irradiates the measurement light toward the calibration area, the calibration reflection portion reflects the measurement light incident on the calibration area, and the light reception portion receives the measurement light reflected by the calibration reflection portion; and the calibration member further has a fourth connection portion detachably attached to the first connection portion of the probe member, and the calibration member is capable of being concurrently mounted to the probe member and the analytical member. a probe member comprising an attachment portion attachable to a flow path wall of a flow path through which an analyte gas flows in a state in which a part thereof is inserted through an opening provided in the flow path wall, an analytical member having a second connection portion detachably attached to a first connection portion located at a base end opposite to a direction of the probe member being inserted into the flow path and located outside of the flow path when attachment portion is connected to the flow path wall and a calibration member that has a third connection portion detachably attached to the second connection portion of the analytical member, a calibration reflection portion, and a calibration area defined therein for introducing the calibration gas, wherein the probe member has a reflective portion and a measurement area defined therein for introducing the analyte gas; the analytical member has a light emission portion and a light reception portion; in a state in which the analytical member is mounted to the probe member, the light emission portion irradiates measurement light toward the measurement area, a reflection portion reflects the measurement light incident on the measurement area, and the light reception portion receives the measurement light reflected by the reflection portion; the probe member has a window portion isolating the measurement area from outside of the base end side and transmitting the measurement light; in a state in which the calibration member is mounted to the analytical member, the light emission portion irradiates the measurement light toward the calibration area, the calibration reflection portion reflects the measurement light incident on the calibration area, and the light reception portion receives the measurement light reflected by the calibration reflection portion; and the calibration member further has a fourth connection portion detachably attached to the first connection portion of the probe member, and the calibration member is capable of being concurrently mounted to the probe member and the analytical member.