In the present invention, in a bottom portion 4a of a lower rail 4, a foreign matter interference portion 27 protruding upward and having a substantially arc-circular shape in a side view is formed by being cut and raised in a middle part in a rail width direction more on a front side than a bolt hole 26 for a mounting bolt for fixing the rear end portion of the lower rail 4 to a vehicle body. Rear-side stoppers 29 and long hole portions 31 shared with the rear-side stoppers 29 are formed on both sides of the foreign matter interference portion 27. The foreign matter interference portion 27 is separated into a front-side tongue piece 27a and a rear-side tongue piece 27b by a slit 32.