The Japanese Ministry of Health, Labour and Welfare issued the basic principles for bioequivalence evaluations of generic dry powder inhaler (DPI) drug products in 2016. This document presents the recommendations of the methodology for the effective development of generic DPI drug products. Based on this document, the Pharmaceuticals and Medical Devices Agency (PMDA) advises the efficient development in the consultation meeting with generic companies. The PMDA generally requires the data of in vitro tests, pharmacokinetics studies, and clinical endpoint studies for generic development. In vitro tests play a critical role in the development of the generic versions because these tests are used to predict the efficacy and safety of other populations on whom clinical endpoint studies have not been conducted. We are aware that some points need further discussion, such as the recommendations for at least four groups of stages (group 1: the induction port and pre-separator, group 2: greater than 5 μm, group 3: ranging from 3 to 5 μm, group 4: ranging from 0.8 to 3 μm) for in vitro tests of the generic DPI products. This article shows the current understanding and recommendations with respect to in vitro tests, particularly for at least four groups of stages.