A differentially driven substrate-integrated cavity-backed slot antenna, based on a single-layer substrate, is proposed in this letter. The radiation part of the antenna is a $2\times 2$ element slot array that is fed by the higher order mode of a substrate-integrated cavity. The resonant mode is excited by a pair of symmetric coupling slots that are transformed from a grounded coplanar waveguide (GCPW) structure. In addition, the antenna bandwidth is enhanced by an additional resonant frequency, which is introduced by the feeding slots. In comparison to a conventional noncoplanar microstrip-slot coupling structure, the GCPW-slot coupling structure can be made coplanar, thereby effectively reducing the structural complexity of the antenna. In addition, the implementation of a balanced feeding scheme suggests that the radiation patterns of the proposed antenna are symmetric in both the E-plane and H-plane with low cross polarization. Moreover, under common excitation, the coupling slots cannot be effectively excited to conduct current, which indicates that the feeding structure has the characteristic of common-mode rejection.