We examine the stiffness of the Heisenberg spin-glass (SG) model at both zero temperature (T = 0) and finite temperatures (T = 0) in three dimensions. We calculate the excess energies at T = 0 which are gained by rotating and reversing all the spins on one surface of the lattice, and find that they increase with the lattice size L. We also calculate the excess free-energies at T = 0 which correspond to these excess energies, and find that they increase with L at low temperatures, while they decrease with increasing L at high temperatures. These results strongly suggest the occurrence of the SG phase at low temperatures. The SG phase transition temperature is estimated to be T SG ∼ 0.19J from the lattice size dependences of these excess free-energies.KEYWORDS: Heisenberg spin-glass, stiffness, excess energy, excess free-energy, rotation, reversal §1. Introduction Spin-glasses have attracted great challenge for computational physics. It has been believed that the spin-glass (SG) phase transition occurs in three dimensions (3d) for the Ising model 1, 2, 3) but not for the XY and Heisenberg models. 4, 5, 6, 7) However, numerical studies during the last decade have revealed that the SG phase might be more stable than what were previously believed. For example, in the isotropic Ruderman-Kittel-Kasuya-Yoshida (RKKY) model, it was suggested that the SG phase transition takes place at a finite temperature. 8, 9) It was also suggested that, for the Ising model, the SG phase might be realized at low temperatures even in two dimensions (2d). 10,11,12,13) For the XY and Heisenberg SG models, Kawamura and coworkers took notice of the chirality described by three neighboring spins, and suggested that, in 3d, a chiral glass phase transition occurs at a finite temperature without the conventional SG order. 14,15,16,17,18) The basis of their suggestion is the stiffness of the system at zero temperature. 14, 15) They estimated θ c > 0 and θ s < 0 from the lattice size dependences of the chiral and the spin domain-wall energies, where θ c and θ s are the stiffness exponent of the chiralities and that of the spins, respectively.