An optical signal transfer device includes a transmitter system and a receiver system for signal transfer between a base unit and a rotary unit. The receiver system includes a natural number n of light receiving portions forming a ring-like light receiving area around the rotary axis and a light reception controller. The transmitter system includes a light emitting portion, an emission controller, and an optical guide which guides a light beam from the light emitting portion to a light emitting area facing the light receiving area. The optical guide guides a light beam to the light emitting area in one of n equally divided areas of a ring-like opposing area which opposes the light receiving area. Therefore, one of the n light receiving portions always face the light emitting area irrespective of rotary position of the receiver system around the rotary axis relative to the transmitter system.
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