There is provided a novel carbonate compound of a nucleoside having a pyrrolopyrimidine skeleton having an excellent antitumor effect, or a pharmaceutically acceptable salt thereof and a method for preventing and/or treating a tumor in combination with an alkylating agent and/or a radiation therapy. According to one aspect of the present invention, there is provided a compound represented by the following general formula (1): or a pharmaceutically acceptable salt thereof, and a method for preventing and/or treating a tumor in combination with an alkylating agent and/or a radiation therapy.