An in-vehicle device fixing structure includes: a base member including a bottom wall, a first side wall, and a second side wall; a box-shaped in-vehicle device including a first side face opposed to the first side wall, a second side face opposed to the second side wall, a third side face provided on a side opposite to the first side face, and a fourth side face provided on a side opposite to the second side face; a holding member including a third-side-face holding portion opposed to the third side face, a fourth-side-face holding portion opposed to the fourth side face, and a fastening portion attached to the bottom wall via a fastener and having a plate shape; and an in-vehicle device locking portion provided in at least the base member and the holding member, and configured to restrain a movement of the in-vehicle device toward the vehicle upper side.