A coating agent that has enhanced durability and a medical instrument (for example, a needle) that has been coated with the coating agent, are provided. The coating agent includes a hydroxyl-group-containing polyorganosiloxane represented by the following general formula (1), a polydiorganosiloxane represented by the following general formula (2), and an amino-group-containing polyorganosiloxane represented by the following general formula (3). The polydiorganosiloxane (2) is contained in a proportion of a mass ratio to the amino-group-containing polyorganosiloxane (3) of from 0.7:1 to 3.0:1. The hydroxyl-group-containing polyorganosiloxane (1) is contained in a proportion of 2.4 to 5.5 mass %, based on the total mass of the hydroxyl-group-containing polyorganosiloxane (1), the polydiorganosiloxane (2) and the amino-group-containing polyorganosiloxane (3).