An object of the present invention is to provide a resonant sensor having excellent sensitivity and selectivity with respect to a component to be detected that is contained at a low concentration in the system. A resonant sensor of the present invention has a receiving layer that contains a polymer having a repeating unit represented by Formula (1). In Formula, R1 represents an alkyl group. A plurality of R1's may be the same as or different from each other. R2 represents a hydrogen atom, an alkyl group, or an aryl group.