Compared to oral medication, the base plays a large role in the external preparation for skin, and dermatologists select the dosage form based on understanding of the effect of the base as well as according to skin symptoms and conditions, application site, age, season, etc. Further, in treatment with external preparations, it is important for patients to understand the application method and continue to apply an adequate amount to achieve the treatment goal. However, there is little evidence regarding the relationship between base properties or usability and the application amount. In this study, we investigated the usability and application amount of three bases with different properties (ointment base, cream base, and lotion base) in 62 adult subjects and exploratively examined the effect of the different base properties on the application amount. The results of this clinical study showed that the usability and preference for the base used for external preparations varies, and poor usability and low preference may lead to a reduction in the application amount. Even with good usability and high preference, there were many cases in which an adequate amount was not applied due to lack of specific instructions on external use. When selecting or changing the base in an external therapy, it is important for not only dermatologists but also pharmacists providing instructions on external use to be aware of the importance of the base and actively instruct patients to apply an adequate amount of the preparation.