A probe unit includes a gas sensor having multiple gas introduction bores for introducing a gas into an inside of the gas sensor, and a sensor holder inside of which the gas sensor is held and that is arranged inside of a duct so as to introduce an exhaust gas flowing in the duct into the gas sensor. A calibration gas flow channel is arranged in the sensor holder and has an opening at an inner surface of a side wall, surrounding the gas introduction bores of the gas sensor, of the sensor holder so as to supply a calibration gas to the gas sensor. A guide groove is arranged continuous to the opening of the calibration gas flow channel and arranged on the inner surface of the side wall along an arranging direction of the multiple gas introduction bores to face the multiple gas introduction bores.