Provided is a crystal of a novel pyrido[3, 4-d]pyrimidine derivative having excellent CDK 4/6 inhibitory activity. A crystal of a compound represented by formula (I). In the formula, R1 represents a hydrogen atom or a C1-3 alkyl group; R2 represents a hydrogen atom or an oxo group; L represents a single bond or a C1-3 alkylene group; and X represents CH or N.