To provide an agent for hypodermic injection that has excellent flow characteristics, can be used particularly even in thin injection needles, for example with an internal diameter of 0.20 mm or less, and is unlikely to cause pain during injection and inflammation. The agent for hypodermic injection includes a hydrogel containing sintered hydroxyapatite particles, wherein the sintered hydroxyapatite particle content is 1 to 60% by mass based on the total mass of the agent, and the average particle diameter of the sintered hydroxyapatite particles is 15 nm or less.
