A method and a device for opening a fiber bundle, capable of performing a fluctuating operation, at a high speed, of pushing a part of a conveyed fiber bundle by a contact member into a stress state and then separating the contact member from the fiber bundle so as to temporarily relax the fiber bundle, and also capable of reducing damage to the fiber bundle. The device for opening a fiber bundle includes a conveying portion 5 for pulling out a fiber bundle Tm from a yarn feeding body 11 and conveying it in a fiber length direction, a fiber-opening processing portion 3 for opening the fiber bundle by moving a fiber in a width direction while bending the fiber by letting a fluid pass through the conveyed fiber bundle Tm, and a fluctuation imparting portion 4 for rotating a contact member 42 in a direction inclined with respect to a conveyance direction while bringing it into contact with the conveyed fiber bundle Tm and pushing a part of the fiber bundle Tm into a stress state, and then separating the contact member 42 from the fiber bundle Tm in the stress state so as to temporarily bring the fiber bundle Tm into a relaxed state.