To provide an acoustic coupler capable of achieving both acoustic characteristics and mechanical characteristics required for ultrasonic imaging. The invention provides a hydrogel preparation method in which a plurality of kinds of polymerizations are performed under reduced pressure, and a gel for an acoustic coupler obtained by the preparation method. For example, a gel for an acoustic coupler includes polyacrylamide having a matrix structure and alginic acid, and the alginic acid is retained in a matrix of the matrix structure of the polyacrylamide.