According to Fisher's Separation Theorem, under perfect capital markets, investment decisions and financing decisions are independent; there is no need to consider the preferences of individual shareholders, and managers maximize shareholder wealth by maximizing firm value. However, under imperfect information markets, due to information imperfections, investment decisions are expected to influence financing decisions, ultimately affecting firm value, resulting in an interdependent relationship between investment and financing decisions. Therefore, this study focuses on the interdependence between financing and investment decisions arising from information market imperfections, and analyzes how the types of investment decisions of Korean listed manufacturing firms—specifically tangible asset investment (tangible fixed assets, etc.) and intangible asset investment (R&D, advertising expenditure, etc.)—affect capital structure, and how profitable firms differ from less profitable ones in their financing behavior. First, three hypotheses were derived based on prior research and literature. The results show that despite differences from prior studies in variable selection, sample selection, and statistical analysis methods, the findings are largely consistent with prior research, though some cases yield contrary results. Regarding Hypothesis I on tangible asset decision variables, tangible fixed asset ratio showed a negative (−) relationship with total debt ratio (including current liabilities), contrary to the predicted positive (+) relationship, but a positive (+) relationship with long-term debt ratio. Since tangible fixed assets are long-term assets, the regression results using long-term debt ratio as the dependent variable are considered more appropriate, and are thus consistent with predictions. For Hypothesis II on intangible investment decision variables, since intangible assets cannot retain their original value when separated from the firm, a negative (−) relationship with debt ratio was predicted. However, R&D ratio showed mixed, contradictory relationships with substantial inter-industry variation. Notably, advertising ratio showed a negative (−) relationship primarily in consumer goods industries selling directly to consumers and in industries with high advertising expenditures such as food and beverages, textiles/apparel/leather, wholesale trade, and transportation/warehousing, although this was not supported by statistical significance. Finally, regarding Hypothesis III, which was derived from Myers's (1984) Pecking Order Theory, the change in working capital ratio and profitability variables showed a negative (−) relationship across all three dependent variables—total debt ratio, long-term debt ratio, and debt-to-equity ratio—consistent with predictions. The regression results for the full sample of firms also showed a statistically significant negative (−) relationship, consistent with the hypothesis.