NOVELTY - Composite separation membrane structure (5) comprises: a support (5a) having a mesh structure; a coating layer (5b) of a porous sol-gel material disposed on the support; a self-assembled monolayer (5c) disposed on the coating layer of the porous sol-gel material.    USE - The composite separation membrane structure is useful in a gas sensor apparatus, which is useful for measuring gas concentration in insulating oil of a power transformer (all claimed), where the gas includes hydrogen, carbon monoxide, acetylene, ethylene or methane gases.    ADVANTAGE - The composite separation membrane structure: allows a real-time monitoring of degradation of insulating oil of a power transformer, hence prevents the breakdown of the power transformer, and increases the service life of the insulating oil; provides the gas sensor apparatus, which allows for easy measurement of a concentration of an individual kind of gas with high reliability using a generally-used semiconductor gas sensor, and does not require an additional mechanical apparatus (such as a vacuum pump for extracting the gases dissolved in the insulating oil), hence it is cost effective; exhibits improved mechanical strength, detection sensitivity, heat resistance and durability; and is prevented from a contamination due to physical or chemical adsorption of degraded insulating oil and water included in the insulating oil.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) a gas sensor apparatus comprising: a semiconductor gas sensor; and the composite separation membrane structure disposed apart from the semiconductor gas sensor;    (2) measuring gas concentration, comprising: obtaining a sensor resistance value by sensing gases dissolved in an insulating oil using at least one semiconductor gas sensor; and obtaining a concentration of an individual gas dissolved in the insulating oil from the sensor resistance value using an equation 1 as given in the specification; and    (3) an apparatus for measuring gas concentration, comprising: at least one semiconductor gas sensor; and a calculation device for obtaining a concentration of an individual gas dissolved in insulating oil using equation 1 as given in the specification from a resistance value, which is obtained by sensing the gases dissolved in the insulating oil using the semiconductor gas sensor.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the composite separation membrane structure.    Composite separation membrane structure (5)    Support (5a)    Coating layer of porous sol-gel material (5b)    Self-assembled monolayer (5c)