Decay-resistant paper suitable for the manufacture of a pot for raising and transplanting seedling is the one partly containing decayable portions, which is prepared by heating paper made of a natural fiber, or a fiber blend composed of a natural fiber and a synthetic fiber, and impregnated with a compound capable of crosslinking the paper cellulose, at a temperature causing the reaction of antidecay treatment and at a temperature causing no such reaction in different portions. Such decay-resistant paper partly containing decayable portions can be easily prepared by bringing the above-mentioned paper impregnated with a compound capable of crosslinking the paper cellulose into contact in a single step with a heating apparatus comprising a heating part of a temperature causing the reaction of antidecay treatment and a heating part of a temperature causing no such reaction. As such a heating apparatus, may be used the one in which a rotary roll heater or a press-type heating plate is coated with a jacket having openings which is made of a material such as iron, copper and aluminum.