p4 Example 3.5: "three critical pairs" should read "three critical pairs (except symmetric one)". (reported by Teppei Saito)
p6 Proof of Lemma 3.12, (4): "by Lemma 3.11(b) where P = { ε }" is unnecessary. (reported by Teppei Saito)
p26 "Ohlebusch [Ohl02] showed ...": This is not stated in [Ohl02]. Instead, please see [S15, Lemma 4.10]. (reported by Fuyuki Kawano)