Disclosed is a method of preparing a high-activity hydrotreating catalyst for hydrocarbon oil, where metals of Group VI (Mo and/or W) and Group VIII (Co and/or Ni) optionally along with phosphorus, and 0.3 to 3.5 molar times of the total molar amount of the metals of particular carboxylic acid(s) (tartaric acid, citric acid, etc.) and 0.1 times or more of the amount necessary for forming the active metals into their sulfides of particular organic sulfur compound(s) (mercapto-acetic acid, mercapto-succinic acid, etc.) are added to a material consisting essentially of at least one of inorganic acids and their hydrates, and they are kneaded shaped and dried at 200.degree. C. or lower. Using the catalyst, hydrotreatment, desulfurization and denitrogenation of hydrocarbon oil may be effected under mild conditions at a reduced operation cost.