To provide a method for producing a positive electrode active material layer for lithium ion battery that can improve durability and internal resistance of lithium ion battery, and particularly lithium ion battery that operates at high voltage. The method for producing positive electrode active material layer for a lithium ion battery includes coating a substrate with positive electrode mixture slurry containing positive electrode active material, first lithium salt, second lithium salt and solvent, and drying off the solvent. First lithium salt is lithium phosphate, the second lithium salt is selected from the group including of lithium carbonate, lithium hydroxide, lithium nitrate, lithium acetate, lithium sulfate and combinations thereof, and the proportion of the second lithium salt with respect to the first lithium salt is 1 to 50 mol % based on the number of lithium atoms.