This study was to obtain basic information needed to develop the effective weed control method for the production of less polluted agricultural products by inducing viability loss of weed seeds in soil with infrared irradiation. Ceramic plates were heated by LPG with the aid of forced air and the infrared produced from plates was used as the heat source for heating soil. The soil heated in this study was sandy loam with four levels of moisture contents (0.5, 5.1, 9.1, 15.0% wb). The temperature distribution was measured at various soil depths when soil was irradiated with infrared for different irradiation time (30, 60, 90 sec). The soil depths with duration time of minimum 3 minutes over $80^circ C$, temperature inducing viability loss of weed seeds, were investigated. When the moisture content of soil was 0.5% and 5.1% wb, the soil depths which can induce viability loss of weed seeds was greatly increased with increasing irradiation time. When 30 seconds of irradiation time was applied on soil with moisture content of 9.1% or 15.0% wb, any depths of soil tested in this study was not reached to the temperature of 8$0^{circ}C$. Generally, the soil depth being needed for viability loss of weed seeds was decreased with increasing moisture content of soil.