A method of producing a partially separated fiber bundle wherein, while a fiber bundle includes a plurality of single fibers travels along a lengthwise direction of the fiber bundle, a separator provided with a plurality of projected parts is penetrated into the fiber bundle to create a separation-processed part, and entangled parts, where the single fibers are interlaced, are formed at contact parts with the projected parts in at least one separation-processed part, thereafter the separator is removed from the fiber bundle, and after passing through an entanglement accumulation part including the entangled parts, the separator is penetrated again into the fiber bundle, characterized in that a separation processing time t1 during being penetrated with the separator and a time t2 from being removed with the separator to being penetrated again into the fiber bundle satisfy Equation (1): 0.03≤t2/(t1+t2)≤0.5.