A selectively excited bimodal interferometer was developed for highly sensitive detection of refractive index changes. Numerical simulation revealed that multimode waveguides with tall, narrow cross-sections are suitable for realizing a highly sensitive bimodal (fundamental and lateral first-order modes) interferometer, owing to deeper penetration of the lateral first-order mode through its side surfaces into the cladding medium. A device with an optimum design was determined by simulation and fabricated by electron-beam lithography using a negative photoresist as the material for the waveguide core. The detection of glucose was demonstrated, and the sensitivity and detection limit was found to be 2547  rad  /  RIU (RIU—refractive index unit) and 9.8  ×  10^−  6  RIU, respectively.