To obtain a semi-insulative GaAs single crystal which is constant in the carbon concn. in a crystal growth direction by a VGF method and VB method.

A crucible 3 contg. GaAs raw materials 5 and B2O3 6 and an oxygen supply source 8 consisting of one or ≥2 compds. selected from a group consisting of As for vapor pressure control and Ga2O3 and As2O3 8 or one or ≥2 compds. selected from a group consisting of carbon monoxide and carbon dioxide are sealed into a quartz ampoule. Crystal growth is executed while the concn. of the gaseous Co in the quartz ampoule is controlled. The execution of the crystal growth while controlling the concn. of the carbon in the crystal growth is made possible. The semi-insulative GaAs single crystal 10 having the uniform carbon concn. is thus obtd. at a high yield.