Provided are an out-of-head localization system, a filter generation device, a method, and a program capable of appropriate processing. A processing device according to an embodiment includes a frequency information acquisition unit that acquires frequency information on the basis of a frequency response of a filter used in filter processing on a reproduction signal, a time information acquisition unit that acquires, on the basis of the frequency information, time information of a reproduction signal, a time signal extraction unit that extracts, on the basis of the time information, a time signal corresponding to at least some time of the reproduction signal, a filter processing unit that performs filter processing on the time signal, and a gain acquisition unit that acquires a gain on the basis of the time signal not having undergone filter processing and a processed time signal having undergone filter processing.