Provided are nickel manganese composite hydroxide particles having a small and uniform particle size and having a double structure which enables to obtain a cathode active material having a hollow structure, and a manufacturing method thereof. When obtaining the nickel manganese composite hydroxide by a reaction crystallization, using an aqueous solution for nucleation, which includes at least a metallic compound that contains nickel, a metallic compound that contains manganese and an ammonium ion donor and controlling the pH value that is measured at a standard solution temperature of 25° C. is 10.5 to 12.0, nucleation is performed in an oxidizing atmosphere in which the oxygen concentration is greater than 1% by volume, and then nuclei are grown by switching the atmosphere from the oxidizing atmosphere to a mixed atmosphere of oxygen and inert gas in which the oxygen concentration is 1% by volume or less.