A process for producing a cyclic diamine which comprises reacting a diamine of the formulaR--HN--R'--NH--R (II)wherein R represents a lower alkyl group and R' represents a lower alkyl group-substituted dimethylene group, trimethylene group, a lower alkyl group-substituted trimethylene group, tetramethylene group or a lower alkyl group-substituted tetramethylene group with urea in the presence of a polar solvent and at 180.degree. C. or higher, to obtain a cyclic urea ##STR1## wherein R and R' are each as defined above, the production yield being more improved by carrying out the initial period reaction at 140.degree. C. or lower.