A transmission system for a hybrid electric vehicle includes a planetary gear mechanism, a gear, a parking lock mechanism, and a resistance imposing device. A parking meshing member of the parking lock mechanism acts so as to switch between a first state where a claw is in mesh with a parking gear and a second state where the claw is out of mesh with the parking gear. The resistance imposing device includes a resistance imposing member configured to impose a rotational resistance on the parking gear or a third rotating element of the planetary gear by coming in contact with the parking gear or the third rotating element. The resistance imposing device is configured to operate the resistance imposing member such that a larger rotational resistance is imposed on the third rotating element in the first state than in the second state.