A positioning drum device is rotatably mounted in positioning base fixedly mounted in a headrail of a Venetian blind and includes a body including a hollow top section having a closed end portion and an open end portion and having a top plate. Two channels are each longitudinally defined in the top section of the body and each terminate at the first closed end portion thereof. Two grooves are each longitudinally defined in the top plate of the top section of the body and each communicate with a corresponding one of the two channels. A plurality of wedge-shaped teeth are formed on each of two sides defining each of the two grooves. Two elongated rods are each disposed in a corresponding one of the two channels and located below an associated groove, and each have one distal end fixedly mounted on the first closed end portion of the top section of the body.