The present invention provides a power back door for a vehicle, in which an opening, through which a connection section between a drive arm and rod of the power back door enters and exits, is covered by a cover member to improve external appearance and in which interference between the connection section and the cover member when the connection section passes through the opening is eliminated to prevent abnormal noise and damage to the cover member during the passage of the connection section. The power back door is a back door (2) for automatically opening and closing a door opening (1). Theconnection section (S), at which the drive arm (3) and rod (4) of the power back door are connected, can enter and exit from door opening (1). A vertically long opening (50) is formed in a side edge of the door opening (1). A pair of cover members (6a, 6b) having lip-shaped forward edges engaging with each other are arranged along left and right vertical edges of the opening (50). The cover members (6a, 6b) are gradually expanded in width so that they have a greater width on the lower side, and this causes the cover members to engage each other on the inner end side of the opening (50). An entrance/exit opening (64) surrounded by the lower edges of the expanded sections of the cover members (6a, 6b) is formed, and the connection section (S) enters and exits from the opening (64).