This study conducted an empirical analysis of the effect of the proportion of outside directors on the board on R&D investment decisions, using a sample of R&D-intensive industries in the United States. The empirical results showed, first, that a higher proportion of outside directors is associated with increased R&D investment, supporting the view that the proportion of outside directors is a structural factor representing board independence. Second, the relationship between the proportion of outside directors and R&D investment was found to be weaker in owner-managed firms than in professionally managed firms, suggesting that in owner-managed firms, the monopolization of decision-making by owner-managers may create information asymmetry problems for outside directors who are excluded from decision-making, which in turn may weaken the board's effective monitoring function.