1. An image encoding method using a depth information, comprising: verifying, by a processor, a depth information of a current coding unit (CU) having a variable size from a depth image: determining, by the processor, whether the current CU has a single object or not based on a size of the current CU and maximum and minimum depth values included in a depth value distribution information of the current CU; determining, by the processor, a motion searching range in a total motion searching region for calculating a motion vector of the current CU based on whether the current CU has the single object or not; and determining, by the processor, an optimum motion vector of the current CU based on a motion information inside the motion searching range.