Provided is a pyrimidine nucleoside compound represented by the following formula (1): or a pharmaceutically acceptable salt thereof, wherein one of X and Y represents a cyano group, and the other represents a hydrogen atom; one of R1 and R2 represents a hydrogen atom, a carbonyl group having a C1-C6 alkyl group which has been mono-substituted by an amino group, or a group represented by (R3)(R4)(R5)Si—, and the other represents a group represented by (R6)(R7)(R8)Si—, or R1 and R2 together form a 6-membered cyclic group represented by —Si(R9)(R10)—; R3, R4, R5, R6, R7, and R8 each represent a C1-C10 linear or branched alkyl group which may have a substituent, a C3-C6 cycloalkyl group which may have a substituent, a C6-C14 aryl group which may have a substituent, or a C1-C6 alkyl group which has been substituted by one or two C6-C14 aryl groups and which may have a substituent; and R9 and R10 each represent a C1-C6 linear or branched alkyl group which may have a substituent. The pyrimidine nucleoside compound of formula (1) or a pharmaceutically acceptable salt thereof exhibits a potent antitumor effect and is therefore useful as a therapeutic agent for preventing or treating a tumor.