The manufacturing efficiency of a semiconductor device is improved. A method for manufacturing a semiconductor device includes a step of sealing a semiconductor chip using a mold die having a cavity, a gate part communicating with the cavity, and a vent part provided opposite to the gate part via the cavity, and extending in a first direction in a sealing step. Further, a lead frame has a first through hole provided at a position overlapping the cavity in the sealing step, and a second through hole provided outside the first through hole, and provided at a position overlapping the vent part in the sealing step. Whereas, in a second direction crossing with the first direction, the length of the second through hole is larger than the length (groove width) of the vent part.