A disperse phase is micronized steadily into liquid droplets of a uniform droplet size. The apparatus therefor comprises a micro-flow channel unit for joining a first fluid for forming a continuous phase and a second fluid for forming a disperse phase together to flow into a single microflow channel having a fine sectional area, and an ultrasonic vibration-applying unit for applying ultrasonic vibration to a microflow channel of the microflow channel unit. With this apparatus, the driving frequency of the ultrasonic vibration-applying unit is made equl to the resonance frequency of the vibration field formed in the microflow channel to enable formation of an emulsion containing ultrafine liquid droplets of the disperse phase. Thereby, the driving frequency of the ultrasonic vibration-applying unit made equal to the resonance frequency of the microflow channel direction enables generation of a powerful ultrasonic wave by a resonance phenomenon, and application of the powerful ultrasonic wave to the microflow channel enables uniform (homogeneous) mixing of the fluid as the continuous phase and the fluid as the disperse phase of the fluid flowing through the micro flow channel to form an emulsion. The disperse phase can be prepared as ultrafine liquid droplets.
