Dried tobacco leaf debris infected with tobacco mosaic virus (TMV) was subjected to heat treatment (6$0^{circ}C$~10$0^{circ}C$) with or without addition of moisture and to room temperature for natural decay to examine the periods of time required for the inactivation of PMV in the inoculum source. Wet conditions (60% moisture content of the debris) for heat treatment were more efficient than dry conditions to inactivate the virus at 7$0^{circ}C$~10$0^{circ}C$, and which decrease of temperature, the time needed for the viral inactivation increased greatly. At 6$0^{circ}C$ and 7$0^{circ}C$, the temperaturein a compost heap during the actively decomposing period, it takes about 15 days or more for the complete inactivation of the virus. However, considering the decrease of the viral infectivity during the decomposition, a shorter period of time will be required to inactivate TMV in the conditions mentioned above, suggesting that a well decomposed organic manure containing tobacco leaf debris may not have infective TMV and may not provide a potential inoculum source.