1. A method for controlling creep torque to prevent a vehicle using an electric motor as a driving source from rolling backward on an upward slope, the method comprising: calculating, by an electric motor controller, in real-time, a gradient of a driving road from a detection value of a sensor and vehicle acceleration; calculating, by the electric motor controller, a maximum creep torque for preventing rolling-back caused by gravity according to the gradient using the gradient and vehicle information; calculating, by the electric motor controller, a first creep torque reference according to a vehicle speed based on the maximum creep torque and the vehicle speed; calculating, by the electric motor controller, a second creep torque reference according to vehicle acceleration based on the maximum creep torque and the vehicle acceleration; calculating, by the electric motor controller, a torque command value according to an operation state of a brake based on the first creep torque reference value and the second creep torque reference value, and the gradient; and generating, by the electric motor controller, a creep torque by controlling a torque output of a driving motor according to the calculated torque command value, wherein calculating the first creep torque reference value includes calculating the first creep torque reference value from a creep torque-vehicle speed map representing a relation between the maximum creep torque and the vehicle speed, wherein in the creep torque-vehicle speed map, a creep torque increment to be applied to a vehicle speed section in which rolling-back of the vehicle occurs and a vehicle speed section to a predetermined creep medium speed in a state where the vehicle acceleration changes into a positive (+) value is set to a value according to the maximum creep torque and the vehicle speed, wherein the step of calculating the torque command value comprises finally determining a minimum value of the first creep torque reference value and the second creep torque reference value as a creep torque reference value, and calculating a torque command value according to a brake depth from a brake depth-torque map representing a relation among the creep torque reference value, the brake depth, and the gradient, and wherein the brake depth-torque map is set such that the brake depth at which the creep torque is released (creep torque equals to 0) when a brake pedal is depressed varies according to the gradient.