An insert detachably mounted in a replaceable-cutting-edge rotary cutting tool, wherein: the insert is provided with an upper surface part having a cutting blade on a ridge line, a lower surface part, a side surface part, and a locking-screw center hole. The cutting blade comprises a corner blade provided on each acute-angle corner part, and a bottom blade and outer peripheral blade provided on the ridge lines on both sides of the corner blade. The bottom blade and the outer peripheral blade have differing clearance angles. The clearance surface of the corner blade has a variable-angle surface part in which the clearance angle varies continuously from the bottom blade side to the outer peripheral blade side. On the upper end of the outer peripheral blade is formed a recessed section defining the length thereof.