A seat locking device includes a striker and a catching section. The catching section includes a base plate including a first entrance concave portion, a cover member including a second entrance concave portion, a latch, a lock plate, and a bumper rubber including a substantially isosceles-triangle portion. A lateral surface of a holding concave portion of the latch is inclined such that an opening side of the lateral surface is located closer to a bottom surface of the first entrance concave portion than its counter-opening side. A bottom surface of the holding concave portion is closer to an imaginary center line of the first entrance concave portion than one lateral surface of the first entrance concave portion so that a catching-capable dimension for the striker is set between the bottom surface of the holding concave portion and another lateral surface of the first entrance concave portion.