Optical sensor unit for transcutaneously measuring blood gas concentration, comprising: - a first unit comprising - sensor means (12, 13), including at least one sensing layer adapted to be irradiated with a predetermined radiation and at least one gas-permeable layer adjacent to one side of the at least one sensing layer and adapted to pass gas, which concentration is to measured, through the gas-permeable layer towards the at least one sensing layer; wherein the optical sensor unit is adapted to measure an optical response of the at least one sensing layer, which optical response depends on the gas concentration; - first thermal insulation means (14, 16) at least partially surrounding said sensor means (12, 13), wherein the first thermal insulation means (14, 16) at least partially surrounds lateral sides of the at least one sensing layer and - first heat conducting means (15, 15a, 15b) at least partially surrounding the first thermal insulation means; and - a second unit connectable and detachable from the first unit, wherein the second unit comprises: - at least one first heating device providing heat to the first heat conducting means (15, 15a, 15b) and - a first temperature sensor for measuring the temperature of the first heating device; characterized in that the first heat conducting means comprise a second temperature sensor provided to measure a temperature of the first heat conducting means for controlling applied heat to the skin. - a first unit comprising - sensor means (12, 13), including at least one sensing layer adapted to be irradiated with a predetermined radiation and at least one gas-permeable layer adjacent to one side of the at least one sensing layer and adapted to pass gas, which concentration is to measured, through the gas-permeable layer towards the at least one sensing layer; wherein the optical sensor unit is adapted to measure an optical response of the at least one sensing layer, which optical response depends on the gas concentration; - first thermal insulation means (14, 16) at least partially surrounding said sensor means (12, 13), wherein the first thermal insulation means (14, 16) at least partially surrounds lateral sides of the at least one sensing layer and - first heat conducting means (15, 15a, 15b) at least partially surrounding the first thermal insulation means; and - sensor means (12, 13), including at least one sensing layer adapted to be irradiated with a predetermined radiation and at least one gas-permeable layer adjacent to one side of the at least one sensing layer and adapted to pass gas, which concentration is to measured, through the gas-permeable layer towards the at least one sensing layer; wherein the optical sensor unit is adapted to measure an optical response of the at least one sensing layer, which optical response depends on the gas concentration; - first thermal insulation means (14, 16) at least partially surrounding said sensor means (12, 13), wherein the first thermal insulation means (14, 16) at least partially surrounds lateral sides of the at least one sensing layer and - first heat conducting means (15, 15a, 15b) at least partially surrounding the first thermal insulation means; and - a second unit connectable and detachable from the first unit, wherein the second unit comprises: - at least one first heating device providing heat to the first heat conducting means (15, 15a, 15b) and - a first temperature sensor for measuring the temperature of the first heating device; - at least one first heating device providing heat to the first heat conducting means (15, 15a, 15b) and - a first temperature sensor for measuring the temperature of the first heating device;