1. A vehicle rear left and right side warning apparatus, comprising: a three-dimensional object detecting unit that detects, by using a pattern recognition technology, a three-dimensional object that exists at rear left and right sides of a vehicle, with respect to images captured by cameras that photograph the rear left and right sides of the vehicle; a warning issuing unit that issues a warning when a relative positional relationship between the vehicle and the three-dimensional object detected by the three-dimensional object detecting unit satisfies a predetermined condition; a driving-lane detecting unit that detects a driving lane of the vehicle; and a recognition-priority setting unit that varies and sets a recognition priority for a type of three-dimensional object that the three-dimensional object detecting unit detects by using the pattern recognition technology, based on the driving lane detected by the driving-lane detecting unit.