NOVELTY - The sensor (1) has an interferometer (30) provided for detecting vibration of a resonance body (11), which enables optical detection of location of a partial surface (116) of the resonance body. The resonance body and the interferometer are arranged on a substrate (50), which comprises semiconductor material. The resonance body is formed of a recess (51) in the substrate. A resonator (70) partially limits certain space for receiving of molecules to be detected. Optical wave guides (32, 42) are integrated on the substrate in silicon-on-insulator technology.    USE - Photo acoustic gas sensor for photo acoustic detection of presence and/or quantity of molecule species in a gas phase for process controlling in an industrial system e.g. heating system, ventilation system and/or air conditioning system and/or for monitoring carbon-di-oxide, oxygen, nitrogen oxide, nitrogen, ethane and hydrogen in an ambient air and/or for monitoring acetone and respiration alcohol content in a respiratory gas and/or for determining combustion value of combustion gas from an internal combustion engine and biogas from a biogas system and/or for monitoring fire protection and/or for analysis of skin perspiration of a living being for detection of acetonaemia for treatment of diabetes and and/or for quality monitoring and/or for monitoring degree of ripeness of a foodstuff e.g. fruits and vegetables, and/or plants in cooling containers (all claimed).    ADVANTAGE - The gas sensor exhibits a compact construction form. The design of the sensor enables acoustic detection of presence and/or quantity of molecule species in the gas phase in a reliable manner. The gas sensor is manufactured in a simple and economical manner.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a method for photo acoustic detection of molecules in a gas phase    (2) a method for manufacturing the gas sensor.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of a gas sensor.    Gas sensor (1)    Resonance body (11)    Interferometer (30)    Optical wave guides (32, 42)    Substrate (50)    Recess (51)    Resonator (70)    Partial surface (116)