To provide a downhole tool that can maintain a high degradation rate even in high-temperature environments and a method for well drilling using the downhole tool. A downhole tool including: a component containing a reactive metal; and a component containing a degradable resin composition promoting degradation of the reactive metal, the degradable resin composition containing a degradable resin producing an acid by degradation, wherein a molar ratio of a maximum amount of the acid which the degradable resin composition is capable of producing to a content of the reactive metal is 1.0 or higher.