A method for positioning a work, which is adapted for a knock hole of a small diameter and which can absorb a pitch error between a main positioning hole and an auxiliary positioning hole of the work is disclosed. An insert shaft of a main positioning device is inserted into the main positioning hole. An auxiliary positioning device includes a base lower block having a parallel slide holding surface, a sliding flange having a sliding surface, and an insert shaft disposed above the sliding flange. The sliding flange of the auxiliary positioning device is guided and moved by the slide holding surface, and is inserted, while absorbing the pitch error, into the auxiliary positioning hole, thereby to position the work.