The present invention relates to a process for producing a hydrate of 3-(2,6-dichlorophenyl)-4-imino-7-[(2′-methyl-2′,3′-dihydro-1′H-spiro[cyclopropane-1,4′-isoquinolin]-7′-yl)amino]-3,4-dihydropyrimido[4,5-d]pyrimidin-2(1H)-one (Compound A) or of a pharmaceutically acceptable salt of Compound A and a crystalline form of Compound A or of a pharmaceutically acceptable salt of Compound A, which are useful in the field of treatment of various cancers as a kinase inhibitor, especially as a Weel kinase inhibitor.