We studied the phase equilibrium of carbon which holds the hydrogen-adsorbed surface at low pressure. All of the molecules in a chamber of a finite volume are assumed to be fully decomposed so that the system is considered to be a mixture of atomic hydrogen and carbon vapor with a crystalline carbon. The equilibrium conditions are characterized by the same chemical potentials for both phases of each ingredient. The phase boundary of a two-dimensional solid with a smaller bond strength is separable from that of a three-dimensional solid with a larger bond strength. The covering ratio of hydrogen atoms on the surface was also calculated and turns out to be larger for a two-dimensional solid.