A former study has shown that the spin-lattice relaxation time (T-1) in cancerous prostate tissue had enhanced contrast at an ultra-low magnetic field, 132 mu T. To study the field dependence and the origin of the contrast we measured T-1 in pairs of ex-vivo prostate tissues at the Earth's magnetic field. A portable and coil-based nuclear magnetic resonance (NMR) system was adopted for T-1 measurements at 40 mu T. The T-1 contrast, delta = 1 - T-1 (more cancer)/T-1(less cancer), was calculated from each pair. Additionally, we performed pathological examinations such as Gleason's score, cell proliferation index, and micro-vessel density (MVD), to quantify correlations between the pathological parameters and T-1 of the cancerous prostate tissues.