A hybrid plasmon waveguide (HPWG) consisting of a gold layer, a 100-nm high nanogap, and a SiN layer with a high refractive index on a SiO₂  /  Si substrate was designed by simulation and was applied in a Mach–Zehnder interferometer biosensor. For the incidence of a transverse magnetic (TM) mode light, a strongly enhanced electric field in the nanogap region caused by the modified surface plasmon enabled highly sensitive detection at the metal/solution interface. In contrast, the transverse electric (TE) mode enabled a high bulk-sensitivity due to the evanescent field in the nanogap. The HPWG structure was formed by electron beam lithography and electrochemical sacrificial layer etching. The output intensity of the fabricated device oscillated upon the replacement of the medium in the nanogap for both TE and TM mode excitations, which agreed with the obtained simulation results. Furthermore, the hybridization of DNA on the gold surface was detected, demonstrating that the HPWG structure is applicable to use in biosensing.