An opening/closing-body drive device is achieved, in which a malfunction, caused by the electrical contacts of the relay switch freezing, can be reliably prevented from occurring. The control unit 60 includes a freeze determiner 61 which determines that the relay switch(es) is frozen when an opening/closing-body drive motor 27a does not drive a specified amount in accordance with opening/closing operational conditions of a back door 3 and when the temperature of the control unit 60 that is detected by a temperature detector 80 is lower or equal to a predetermined value; and a freeze releaser 62 which releases a frozen state of the relay switch(es) by intermittently supplying electric current for a predetermined number of times to the relay switch(es) without supplying a drive current to said opening/closing-body drive motor 27a, when said freeze determiner 61 determines that the relay switch(es) is frozen.