To solve the problem with prior art that, in a prior art fire power adjustment apparatus for gas instruments left standing for a long time to cause its interior to be replaced by air, if a gas burner is fired and just thereafter a rapid drawing operation is performed, air in a downstream chamber 12b flows through a gas passage 11b located on a downstream side so that flame failure can occur to gas burner due to excess air.

A gas passage 11b located in the downstream of a movable valve 14 is formed such that there is partly formed a region where pressure in a gas passage 11b is lowered when fuel gas is made to flow. Connection between the gas passage and a lower chamber 12b of a chamber 12 is achieved through at least two communication passages 17a, 17b which are opened in the region where pressure is lowered and on an upstream side of the foregoing region, and which are isolated from each other.