Internal domain 3 of seven transmembrane G protein-coupled receptors is important for G protein binding or receptor activity, and is well conserved. In H3 receptors, which are a type of G protein-coupled receptor, this region is also conserved in the same way. Therefore, as a result of using PCR to introduce point mutations into sequences encoding the region in H3 receptor cDNA, H3 receptor mutants comprising extremely strong constitutive activity could be successfully produced. The present inventors further found that by using constitutively active H3 receptor mutants, drug candidate compounds such as H3 receptor inverse agonists can be screened more easily and efficiently.