A sensation of presence of voice chat in a virtual space is enhanced. A user speech synthesizer used in a virtual space sharing system where information processing devices share the virtual space. The user speech synthesizer comprises a speech data acquiring section (60) for acquiring speech data representing a speech uttered by the user of one of the information processing devices, an environment sound storage section (66) for storing an environment sound associated with one or more regions defined in the virtual space, a region specifying section (64) for specifying a region corresponding to the user in the virtual space, and an environment sound synthesizing section (68) for acquiring the environment sound associated with the specified region from the environment sound storage section (66), combining the acquired environment sound and the speech data and synthesizing synthesized speech data.