We measured the temperature dependence of the electrical resistivity for two high-purity undoped 6H-SiC bulk wafers with resistivities of 1.5103 cm and 8.3108 cm at room temperature. We also characterized the deep levels affecting the semi-insulating property by current deep level transient spectroscopy (I-DLTS) and photo induced current level transient spectroscopy (PICTS) measurements. The activation energies of the resistivity were 0.11 eV and 0.59 eV for the samples with lower and higher resistivities, respectively. In I-DLTS and PICTS spectra, the sample with lower resistivity shows a donor level at Ec0.17 eV and two acceptor levels around Ec0.40 eV, while the sample with higher resistivity shows acceptor levels at Ec0.77 eV and Ev+0.46 eV. We calculated the temperature dependence of the resistivity with a model considering one donor level and one acceptor level based on parameters from I-DLTS peaks. We reproduced the experimental results only for the sample with lower resistivity. The acceptor level near the valence band needs to be considered to explain the resistivity for the sample with higher resistivity.