In establishing a market for Vinylon, a domestically developed synthetic fiber, it was important not only to continue improving the technology, but also to focus on applications that could demonstrate practicality through product development and advertising that incorporated marketing ideas. Soichiro Ohara, the president Kurashiki Rayon, has been credited with leading the company to success by shifting its thinking to match the market and technical conditions for Vinylon, and by demonstrating strong leadership. However, in reality, Soichiro Ohara was distressed by Vinylon’s poor performance. He tried to manage the situation rationally to prevent Vinylon from failing, all while grappling with various thoughts and emotions. This paper does not seek to undermine Soichiro Ohara’s reputation, but rather, by using his diary, which was recently discovered, attempts to look into the feelings of a manager who was torn between emotion and rationality in relation to Vinylon. Since the study of business history attempts to read management decisions and actions objectively based on market, technological and financial rationality, and so can be said to neutralize the emotions of managers. While emotions do not always steer things in the right or desirable direction, they can influence managerial decisions and actions that appear rational. Therefore, I believe that it is necessary to pay attention to the role and effects of emotions. This paper attempts to decipher the words of Soichiro Ohara in his diary, in which he could freely express his emotions, by approaching them from the perspective of the “history of emotions,” focusing not only on rationality but also on emotions.