The present invention relates to a liquid-phase composition containing alginic acid or a pharmaceutically acceptable salt thereof and a colloidal polysaccharide, the composition being suitable as a medical biomaterial for tissue restoration used to elevate mucosal tissue of stomach, bowels, or the like when the mucosal tissue is subjected to endoscopic excision, and more specifically, to a liquid-phase composition, which is a medical pharmaceutical composition to be injected into the submucosal layer in endoscopic mucosal resection (EMR) or endoscopic submucosal dissection (ESD), wherein the liquid-phase composition contains alginic acid having a weight average molecular weight of 100,000 to less than 200,000 or a pharmaceutically acceptable salt thereof, and a colloidal polysaccharide. According to the present invention, the liquid-phase composition can be effectively used as a local submucosal injection agent which superbly sustains injection-induced elevation of mucous membranes during endoscopic mucosal resection or endoscopic submucosal dissection.
