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.提供一種具有優異之CDK4/6抑制活性的新穎之吡啶并[3,4-d]嘧啶衍生物的結晶。 式(I)所示之化合物的結晶。式中,R1表示氫原子或C1-3烷基,R2表示氫原子或側氧基,L表示單鍵或C1-3伸烷基,X表示CH或N。