The longitudinal electric field produced by focusing a radially polarized beam is applied in confocal laser scanning microscopy by introducing a higher-order transverse mode, combined with a technique of polarization conversion for signal detection. This technique improves signal detection corresponding to the longitudinally polarized field under a small confocal pinhole, enabling full utilization of the small focal spot characteristic of the longitudinal field. Detailed numerical and experimental studies demonstrate the enhanced spatial resolution in confocal imaging that detects a scattering signal using a higher-order radially polarized beam. Our method can be widely applied in various imaging techniques that detect coherent signals such as second-harmonic generation microscopy.