Informal finance exists extensively and has been playing an important role in small- and medium-sized enterprise (SME) financing in developing economies. This paper tries to rationalize the extensiveness of informal finance. SME financing suffers more serious information asymmetry to the extent that most SMEs are more opaque and can only provide less collateral. Informal lenders have an advantage over formal financial institutions in collecting “soft information” about SME borrowers. This paper establishes a model including formal and informal lenders and high- and low-risk borrowers with or without sufficient collateral and shows that the credit market in which informal finance is eliminated will allocate funds in some inefficient way, and the efficiency of allocating credit funds can be improved once informal finance is allowed to coexist with formal finance.