It is known from observations that stress drop of intraplate earthquakes is larger than that of interplate earthquakes on average. I present a mechanical model for explaining the difference in stress drop, where an intraplate fault is assumed to be loaded uniformly, while a locked area (an asperity) on a plate boundary is nonuniformly loaded by surrounding aseismic sliding. A numerical simulation is conducted to quantitatively evaluate the difference in stress drop for different loading processes. Because the model intraplate fault is uniformly loaded, stress drop is uniform over the fault with a larger average value. In contrast, stress concentration is generated at a boundary region between locked area and aseismic sliding area for a model interplate earthquake. This stress concentration hastens earthquake occurrence, resulting in lower average stress drop. The stress drop of simulated intraplate earthquakes is several tens percent larger than that of simulated interplate earthquakes.