Provided is a collagen sponge which has compressive strength (stress) equivalent to that of a tissue into which the collagen sponge is to be implanted, has no unevenness in structure and stress, and has a pore structure for allowing cells to infiltrate thereinto. The collagen sponge is obtained by subjecting a collagen dispersion, a collagen solution, or a mixture thereof having a collagen concentration of 50 mg/ml or more to freeze-drying and insolubilization treatment thereafter. The collagen sponge thus obtained has a stress of from 10 kPa to 30 kPa when loaded with 10% strain, has in its surface and inside a pore structure having a mean pore diameter ranging from 50 μm to 400 μm, and has a pore diameter standard deviation equal to or less than 80% of the mean pore diameter.