It is intended to provide a gene encoding a protein having an activity of improving a tolerance to an environmental stress such as salt, moisture, or heat stress, in particular, a tolerance to water stress; a protein having an activity of improving a tolerance to an environmental stress; a transgenic plant having an enhanced tolerance to an environmental stress, and so on. A halophyte Salicornia europaea, which has a strong activity of improving a tolerance to water stress and can grow even in a dry area with much salt accumulation, was used to search genes participating in the salt tolerance of Salicornia europaea by a functional screening method using an Escherichia coli gene expression system, with the aim of isolating genes (cDNAs) participating in the salt-tolerance mechanism of Salicornia europaea. As a result, it was found out that an arabinogalactan protein consisting of 427 amino acids and having partial homology to a Fasciclin-like arabinogalactan protein of Arabidopsis thaliana has a function of enhancing salt tolerance, water stress tolerance (sorbitol tolerance), and heat tolerance.