In general, the rates of released elastic strain energy, kinetic energy and work done required for creating new surfaces are the determinate factors governing the propagation of brittle fracture. In the previous report, the dynamical stress field in a plate having an enlarging slit at a constant speed subjected to a uniform tensile stress normal to the direction of the crack was obtained. In this report, the release rate of elastic strain energy in a plate having an enlarging slit at a constant speed was calculated by using the dynamical stresses obtained in the previous report. The release rate of elastic strain energy decreases with the crack speed ratio.