We propose a novel hollow waveguide optical switch composed of a multi-mode interference (MMI) coupler with a variable air core. The numerical simulation of the proposed optical switch is carried out for investigating switch performances. Switching operation can be obtained by mechanical displacement of the air core of an MMI hollow waveguide. A hollow waveguide consists of Ta₂O₅/SiO₂ multilayer mirrors for optical confinement. The result shows a possibility of switching of about 99.5% optical power fraction with a switching length of 500µm and a small displacement (ΔD_core=2.5µm) of an air core thickness.