The invention relates to a process for preparation of 4,6-diaminoresorcinol or salts thereof by reducing 4,6-bis(substituted)phenylazoresorcinol expressed by the formula [1] wherein, R denotes a halogen atom, an alkyl group having 1-5 carbon atoms, a hydroxycarbonyl group or an alkoxy group having 1-5 carbon atoms, n denotes 0 or any integer of 1-5, and two or more groups R may be same or different each other, for example, 4,6-bisphenylazoresorcinol, with hydrogen in the presence of a metal catalyst to obtain 4,6-diaminoresorcinol or salts thereof, characterized in that an aliphatic nitrile compound (for example, acetonitrile) is used as a solvent, or characterized in that the reduction is carried out by using at least one organic solvent selected from aliphatic nitrile compounds, aliphatic alcohols having 3-5 carbon atoms, dioxane and ethylene glycol monomethyl ether, etc. as a solvent and furthermore in the presence of a filter aid (for example, active carbon). According to the said preparation process, 4,6-diaminoresorcinol with high purity can be easily obtained from 4,6-bis(substituted)phenylazoresorcinol in a high yield.