A gas sensor unit including a gas sensor and a sensor cap. The gas sensor includes a gas detecting element, an electrode provided on the gas detecting element and a sensor terminal connecting to the electrode for transmitting an output signal from the gas detecting element. The sensor cap is adapted for transmitting the output signal to an external device. The sensor cap includes a cap terminal electrically connecting to the sensor terminal and an enclosing member bonding to the gas sensor so as to form an internal space in cooperation with the gas sensor. The enclosing member includes a communicating hole through which the internal space is in communication with a space outside the gas sensor unit, and the communicating hole is gas-permeably and watertightly closed by a filter member.