To provide a weight increase-improving agent for crustacea, which has high safety, is inexpensive and stably supplied.

The weight increase-improving agent for crustacea effectively contains a 2-6C organic acid or its salt. The weight increase-improving agent may effectively contain the 2-6C organic acid within crustacea feed. The content of the 2-6C organic acid or its salt accounts for about 0.001-10 wt.% for example. The 2-6C organic acid or its salt preferably is propionic acid or its salt. The crustacea more preferably is shrimp, particularly preferably is prawn, or especially preferably is giant tiger prawn.