An externally toothed gear of a dual-type strain wave gearing is provided with first and second external teeth having different teeth numbers, and a gap formed between these teeth as a cutter clearance area for tooth cutters. Where L1 is the maximum width of the gap, t1 is a depth from the tooth top land of the first external teeth to the deepest part of the gap, h1 is the tooth depth of the first external teeth, t2 is a depth from the tooth top land of the second external teeth to the deepest part, and h2 is the tooth depth of the second external teeth, any one of the following conditions 1 to 3 is satisfied: L1=0.1L−0.35L, t1=0.9h1−1.3h1, and t2=0.3h2−0.9h2  Condition 1: L1=0.1L−0.35L, t1=0.3h1−0.9h1, and t2=0.9h2−1.3h2  Condition 2: L1=0.1L−0.35L, t1=0.3h1−0.9h1, and t2=0.3h2−0.9h2  Condition 3: It is possible to obtain a dual-type strain wave gearing in which wear resistance and tooth bottom fatigue strength are increased.