Microspheres (MS), such as emulsion droplets, multiple emulsions, microparticles, microcapsules, and liposomes, have been utilized in various industries. However, size control of MS is not so easy. Recently, we proposed a novel method for preparing monodispersed emulsion droplets with a coefficient of variation less than 5% from a microfabricated channel (MC) array. In this study, we analyzed a droplet-formation mechanism from a MC using a microscope high-speed camera system. During droplet formation, the dispersed phase passed through the channel inflated on the terrace in a disklike shape, and the droplets were formed in 0.01 s. A droplet-formation mechanism was suggested in which the distorted dispersed phase on the terrace is cut off spontaneously into spherical droplets by interfacial tension. The mechanism is shown to be an adequate model from the viewpoint of interfacial free energy. This emulsification technique exploits the interfacial tension, which is the dominating force on a micrometer scale. It is a promising technique for producing MS requiring extreme monodispersity because of its simplicity.