A gel for an ultrasonic phantom is provided which has satisfactory ultrasonic characteristics, does not fluctuate in ultrasonic characteristics over long, and is free from germ multiplication, alteration, putrefaction, etc. The gel is a gel for ultrasonic phantoms which comprises: a segmented polyurethane gel having alkylene oxide segments which are mostly or wholly liquid at ordinary temperature; and a nonvolatile organic gel-swelling medium not participating in a urethane-forming reaction, the medium being contained in the segmented polyurethane gel. Due to the nonvolatile organic gel-swelling medium, the liquid segments and the like are kept in an extended state and the distance between the segments is increased. The gel hence has satisfactory ultrasonic characteristics concerning ultrasonic attenuation rate, propagation velocity, etc. The gel does not change in swelling-medium content with time and hence retains the satisfactory ultrasonic characteristics over long. Furthermore, the gel is free from germ multiplication, alteration, putrefaction, etc. because of the organic gel-swelling medium contained therein.