A temporary plugging agent for well drilling having a plugging function for a period of not greater than 40 days in a high-temperature environment having a temperature range between 93 and 204° C., and preferably, the above temporary plugging agent for well drilling (optionally formed from two or more types of plugging agents for well drilling) wherein a contained synthetic resin has a ratio of mass loss rate in 80° C. deionized water relative to the mass loss rate of polyglycolic acid of not less than 0.001 and less than 1, and/or has a compressive strength after a period of not greater than 40 days has elapsed at a temperature range between 93° C. and 204° C. of not less than 20% lower than the compressive strength before the period began; a well-treatment fluid comprising the temporary plugging agent for well drilling; and a method for well drilling in which a temporary plugging treatment is performed using the temporary plugging agent for well drilling.