In an aspect, a sensor includes a combining portion that combines with a second substance having a molecular weight larger than a molecular weight of a first substance. Further, in an aspect, the sensor includes a substrate including a surface on which the combining portion is disposed. The combining portion detects whether or not the first substance is included in an analyte that has come into contact with both an aptamer and the second substance. The aptamer includes a first combining part for the first substance and a second combining part for the second substance and is combined with either of the first substance and the second substance.