A medical dilating instrument (100) includes a needle body (10) having a pointed needle tip (11), an elongated guide portion (30) formed so that a tip (31) thereof is duller than the needle tip (11), and a trunk portion (50) having a larger diameter than the guide portion (30). A cannular needle (20) is configured by connecting the needle body (10) and the guide portion (30), and a dilator (40) is configured by connecting the guide portion (30) and the trunk portion (50). The dilator (40) includes a diameter-enlarging portion (42) that increases in diameter from a tip end side toward a base end side. The diameter-enlarging portion (42) is provided on one or both of the guide portion (30) and the trunk portion (50).
