A keyboard instrument including an instrument main body having, on one side surface thereof, a first fitting section including a first fitting convex section, a first fitting concave section that is more concave than the first fitting convex section, and a first guide section provided therebetween, and a side plate which supports the one side surface of the instrument main body and has a second fitting section including a second fitting convex section corresponding to the first fitting concave section, a second fitting concave section corresponding to the first fitting convex section, and a second guide section provided therebetween so as to correspond to the first guide section, in which a vertical length of a rear side of the second fitting concave section in the side plate is larger than a vertical length of a front side of the second fitting concave section in the side plate.