PROBLEM TO BE SOLVED: To provide a method of producing microsphere excellent in stability and pressure resistance by an ultrasonic or a mehcanical cavitation process which improves a conventional method of producing microsphere. SOLUTION: The method of producing microsphere of an encapsulated gas useful for an ultrasonic imaging agent includes the steps of a) a process of preparing a protein solution which can thermally be denaturated at a temperature required for achieving an initial denaturation temperature in the next mechanical emulsification, b) a process of joining the solution and the gas together, c) a process wherein the mixture of the protein solution and the gas is emulsified by mechanically shearing the mixture and forming a susprnsion comprising minute bubbles of the gas having an average diameter in the range of approximately 0.1 μ to approximately 10 μ, and d) a process wherein the minute bubbles of the gas areas are encapsulated and the microspheres are formed by subjecting the suspension to the mechanical cavitation and denaturing the protein to be deposited on an interface between the gas and the suspension.