PURPOSE: To provide a resin sealed semiconductor device suitable for a high density mounting to meet an increased number of electrodes without expanding its mounting area, and its mounting structure.

CONSTITUTION: Lead layers isolated from each other are formed in the locations other than electrodes 8 to 10 on the electrode formation side of a semiconductor element 1. At this juncture, multi-layered leads 2 to 7 are formed stepwise where the inner leads become shorter as they are arranged on the upper layers at their leading ends. The connecting parts to the electrodes on the inner leads 2 to 4 of each layer and the electrodes 8 to 10 of the semiconductor element are electrically connected through wires 11 to 13, respectively. Also, the outer leads extended outside the package are formed stepwise where the leads become longer as they are arranged on the upper layers.