A premixing apparatus has: a fan; an air tube; and a gas tube inside the air tube; provided that a direction toward an air flow upstream side inside the air tube is defined as an X+ direction, an air adjusting valve facing an air inlet port positioned at an end, in the X+ direction, of the air tube; a gas adjusting valve facing a gas outlet port positioned at one end of the gas tube; and a common actuator driving the air adjusting valve and the gas adjusting valve. At an end, in the X− direction, of the gas tube, is disposed a valve seat having formed therein the gas outlet port, and a gas adjusting valve inside the gas tube. A valve stem penetrates through an end wall, in the X+ direction, of the gas tube. The gas adjusting valve is driven by the actuator through the valve stem.