Diurea derivatives are provided which can be represented by the following general formula and salts thereof: ##STR1## wherein, R.sup.1 and R.sup.2 are the same or different, and each represents C.sup.1 -10 alkyl group, a C.sup.3 -18 cycloalkyl group, or a lower alkyl group substituted by cycloalkyl radical(s);R.sup.3, R.sup.4, R.sup.5 and R.sup.6 are the same or different, and each represents a hydrogen atom, a lower alkyl group, a cycloalkyl group, hydrocarbyl aralkyl group, a pyridyl group, or an unsubstituted or substituted phenyl group, the substituents of the phenyl group being selected from a group consisting of lower alkyl radical, halogen atom-substituted lower alkyl radical, halogen atom, nitro radical, amino radical, mono- or di-lower alkylamino radical, lower acylamino radical, hydroxyl radical, lower alkoxy radical and lower acyloxy radical;X represents an oxygen atom or a sulfur atom, and n.sup.1 and n.sup.2 represent an integer of 1 to 6; and wherein at least one of R.sup.3 -R.sup.6 is a phenyl substituted by acyloxy.