This invention relates to a pharmaceutical composition for treatment of a bone disease comprising, as an active ingredient, a protein comprising an extracellular cysteine-rich domain, which is from the Frizzled receptor selected from the group consisting of mammalian animal-derived Frizzled 1, Frizzled 2, and Frizzled 7 and has activity of increasing bone mass, bone density, and/or bone strength, or a mutant of such domain having sequence identity of 85% or higher to the amino acid sequence of the domain and having activity of increasing bone mass, bone density, and/or bone strength, or a vector comprising a nucleic acid encoding the protein.