The use of a composition that has a low endotoxin monovalent metal salt of alginic acid having a molecular weight of 500,000 or more and a bone marrow mesenchymal stem cell for treating a cartilage injury. The composition has a viscosity of 400 millipascal-second to 20,000 millipascal-second. The monovalent metal salt of alginic acid can be sodium alginate or potassium alginate and the composition can be cured at the site of injury with the application of calcium chloride.