Small-size directional coupler type optical multi/demultiplexers using high refractive-index-difference composite-glass waveguides have been investigated. A novel design to minimize wavelength response sensitivity to waveguide width deviation in the coupler part was proposed. Composite glass waveguides with 5% were fabricated using Corning 7059 glass for the core and SiO2 for the cladding. 1.3/1.55-µm optical multi/demultiplexers with short coupler length of 300 µm, low insertion loss of 5 dB and high crosstalk attenuation of more than 25 dB were realized.