This paper investigates the determinants of the use of collateral and personal guarantees in Japan's SME loan market. We find that firms' riskiness does not have a significant effect on the likelihood that collateral is used. We find, however, that main banks whose claims are collateralized monitor borrowers more intensively and that borrowers who have a long-term relationship with their main banks are more likely to pledge collateral. These findings are consistent with the theory that the use of collateral is effective in raising the bank's seniority and enhances its screening and monitoring. This incentive effect for the bank becomes tenuous for personal guarantees.