Disclosed is a coating agent that contains a polyorganosiloxane (1) represented by the following general formula (1), a polydiorganosiloxane (2) represented by the following general formula (2), and either one of an amino group-containing polyorganosiloxane (3) represented by the following general formula (3) and a hydroxy group-containing polyorganosiloxane (4) represented by the following general formula (4). A coating agent having improved piecing properties and a medical instrument (for example, a needle) surface-treated (coated) with the coating agent are provided.
