Disclosed is a method of producing high-concentration collagen for use as a medical material, including: washing tissue of a mammal; subjecting the washed tissue to crushing and immersion in ethyl alcohol; subjecting the tissue to enzymatic treatment with stirring in purified water containing phosphoric acid and pepsin; adding sodium chloride to the collagen subjected to enzymatic treatment, performing stirring, and aggregating collagen; dissolving the aggregated collagen in purified water to give a collagen solution, which is then filtered using a filter and concentrated by removing the pepsin, low-molecular-weight material, and sodium chloride from the collagen solution using a tangential flow filtration device; subjecting the concentrated collagen to sterile filtration, aggregating the collagen using a pH solution in a neutralization tank, and concentrating the collagen by removing a non-aggregated solution; and concentrating the concentrated collagen using a centrifuge and stirring the concentrated collagen using a mixer.