A small-size reliable gas sensor that can reduce a measurement error can be provided. The gas sensor includes a first light source; a first sensor unit and a second sensor unit disposed to receive light output from the first light source; a first substrate having a first principal surface on which the first light source and the first sensor unit are provided; and a second substrate having a first principal surface on which the second sensor unit is provided. The first sensor unit is disposed at a location where light output from the first light source and reflected on the second principal surface strikes the first principal surface of the first substrate.