A rear wheel steering apparatus for a vehicle includes a housing (1) including a cylinder portion (10) and a casing (12), a linear motion mechanism (3c) including a nut member (36) and a rod (2), the casing including at least one opening portion (12x, 12y) that opens in parallel to the rod and a communication hole (H) provided separately from the opening portion, the casing positioned surrounding the rod extending from the cylinder portion and being fixed to the cylinder portion, a cover member (15, 16) joined to the casing to seal the opening portion of the casing, a controller (4) housed in the casing, a power line (80a, 80b) and a signal line (90) electrically connected to the controller in the casing, and a thermal shrinkage tube (100) surrounding the power line and the signal line to retain thereof. The power line and the signal line are liquid-tightly supported in the communication hole of the casing via the thermal shrinkage tube.