The present invention provides a composition for replenishing the nucleus pulposus of an intervertebral disc, the composition containing a monovalent metal salt of a low endotoxin alginic acid. The composition is applied to a nucleus pulposus site of a subject, is used to harden a part after application, and has fluidity when applied to the nucleus pulposus site. Accordingly, a composition for replenishing nucleus pulposus is provided, the composition being capable of promoting the regeneration of the nucleus pulposus of an intervertebral disc.