An object of the present invention is to provide an environmentally friendly drilling fluid that has excellent cuttings transport capability and excellent hole cleaning capability. A drilling fluid according to the present disclosure is a drilling fluid containing water, biodegradable fibers, and a thickener. The thickener contains a biodegradable polysaccharide. The content of the biodegradable polysaccharide in the drilling fluid is greater than or equal to 0.01 g/L and less than or equal to 5 g/L.