A power supply apparatus for a vehicular sliding door is provided, including a power supply line electrically coupled to a power supply; a power supply line protector including a rectangular-shaped case and disposed in the vehicular sliding door, in which the power supply line is housed in a bent state; a power supply line path in the case which controls the bent line; a slider received in, and slidable along, a length of the case; an extension spring by which the slider is urged toward one end side of the case; and a slit-shaped opening in an underside of the case; wherein a side portion of the line is drawn out through the slit-shaped opening and secured to a vehicle body side; and wherein a power supply line portion, which has been drawn out from another end side of the case, is connected to appliances inside the door.