A seat cushion extension apparatus includes an extension motor supported on a cushion frame, an extension mounting bracket holding an extension cushion such that the extension cushion is displaceable forward and backward on a seat cushion, and a power transmission unit connecting between the extension motor and the extension mounting bracket. The power transmission unit receives power from the extension motor and moves the extension mounting bracket forward and backward. A slide device is coupled with the extension mounting bracket, and supports the extension mounting bracket such that the extension mounting bracket is slidable forward and backward in the cushion frame. The front-back length of the seat cushion can be freely extended, and limitations in the thickness of a cushion pad or the like are removed.