The extraction of geothermal energy from HDR (hot dry rock) in the earth's crust has received wide attention. An abundant amount of geothermal energy could be recovered from HDR by circulating fluid through crack-like reservoirs which are created by a hydraulic fracturing technique. During the extraction of geothermal energy, the surface of the crack-like reservoirs are cooled by the fluid and the thermal contraction of the rock occurs. As a consequence, secondary cracks may initiate from the crack-like reservoir perpendicular to the reservoir surface. In order to discuss the initiation of the secondary cracks, the present paper is concerned with the stresses around the reservoir including thermal stresses on the basis of theory of quasi-static thermoelasticity. The surrounding rock is assumed to be a continuum which is homogeneous and isotropic with respect to thermal and elastic constants. The singular equations were derived from the boundary conditions and they were solved by means of the combination of the collocation method and the inversion formula. The thermal stresses around the reservoir were shown in graphs and the initiation of the secondary cracks was discussed.