A vehicle door support device includes a support member and a support member that hold a door at an open position. Each of these support members includes: a cylindrical accommodating portion; a cylindrical cover connected to the accommodating portion; a cylindrical movable member movable relative to the cover in an axial direction; a gear mechanism arranged in the accommodating portion; a spindle connected to a first connecting portion of the gear mechanism; a rotation mechanism that rotates the spindle by movement of the movable member relative to the cover; and a coil spring that biases the movable member so as to advance with respect to the cover. At least the support member includes a magnetoresistive mechanism that is arranged in the accommodating portion, is connected to the second connecting portion of the gear mechanism, and applies a load to the spindle via the gear mechanism.