A supply air system comprising a supply air apparatus and an outer wall of an indoor space (ST; ST1) of a building comprising window structures (i1, i2) with a space (ST;ST2) therebetween, the supply air apparatus comprising: an electrical blower arrangement, such as one or more, preferably on axial principle operated blowers, for producing outdoor air into said indoor space in the building by simultaneously mixing the outdoor air (u) to be brought into said indoor space (ST; ST1) with air from the indoor space in order to achieve an air flow (s) passing upwards along a window in the outer wall of the building, wherein the supply air apparatus comprises in a built-in manner: - a box-structured frame; - an internal flow space along with its flow connections for outdoor air and indoor air for mixing the above with each other in the frame; an inflow arrangement of outdoor and indoor air that joins with the flow space and opens into the indoor space; and - a blow flow arrangement that leads the mixed air (us) into said indoor space (ST; ST1), - the supply air apparatus being arranged in connection with an outdoor air flow way that exists in said outer wall of said building, characterized in that, - the supply air system comprises a replacement air valve in connection with said window structures (i1, i2) or above the window structures (i1, i2) in the outer wall of the building, - the supply air apparatus comprises an air distribution device to be placed separately in connection with said replacement air valve, wherein said air distribution device is arranged in said indoor space (ST; ST1) in said building on an inner surface of said outer wall or on a ceiling above said replacement air valve, or in a space (ST; ST2) between the window structures (i1, i2) of the window having said replacement air valve. an electrical blower arrangement, such as one or more, preferably on axial principle operated blowers, for producing outdoor air into said indoor space in the building by simultaneously mixing the outdoor air (u) to be brought into said indoor space (ST; ST1) with air from the indoor space in order to achieve an air flow (s) passing upwards along a window in the outer wall of the building, wherein the supply air apparatus comprises in a built-in manner: - a box-structured frame; - an internal flow space along with its flow connections for outdoor air and indoor air for mixing the above with each other in the frame; - a box-structured frame; - an internal flow space along with its flow connections for outdoor air and indoor air for mixing the above with each other in the frame; an inflow arrangement of outdoor and indoor air that joins with the flow space and opens into the indoor space; and - a blow flow arrangement that leads the mixed air (us) into said indoor space (ST; ST1), - the supply air apparatus being arranged in connection with an outdoor air flow way that exists in said outer wall of said building, - a blow flow arrangement that leads the mixed air (us) into said indoor space (ST; ST1), - the supply air apparatus being arranged in connection with an outdoor air flow way that exists in said outer wall of said building, characterized in that, - the supply air system comprises a replacement air valve in connection with said window structures (i1, i2) or above the window structures (i1, i2) in the outer wall of the building, - the supply air apparatus comprises an air distribution device to be placed separately in connection with said replacement air valve, wherein said air distribution device is arranged in said indoor space (ST; ST1) in said building on an inner surface of said outer wall or on a ceiling above said replacement air valve, or in a space (ST; ST2) between the window structures (i1, i2) of the window having said replacement air valve. - the supply air system comprises a replacement air valve in connection with said window structures (i1, i2) or above the window structures (i1, i2) in the outer wall of the building, - the supply air apparatus comprises an air distribution device to be placed separately in connection with said replacement air valve, wherein said air distribution device is arranged in said indoor space (ST; ST1) in said building on an inner surface of said outer wall or on a ceiling above said replacement air valve, or in a space (ST; ST2) between the window structures (i1, i2) of the window having said replacement air valve.