A gas mixing apparatus including a chamber, a filter, a gas transmitting unit and a porous layer is provided. The chamber includes a gas inlet and a gas outlet. The filter, which has at least one opening, is disposed at the gas outlet of the chamber. An environmental gas outside the chamber is filtered and becomes a clean gas after flowing through the filter into the chamber. The gas transmitting unit is connected to the opening of the filter to transmit a gas to be mixed into the chamber through the opening. The porous layer is disposed at the gas outlet. The gas to be mixed is mixed with the clean gas and leaves the chamber through the porous layer.