To provide an insert for a headrest capable of retaining the quality stability and improving the productivity by reducing working burden when assembling pins with bosses and securing the fitting and integration between the pins and the bosses.

This insert for headrests is so formed that a pin 24 having a projection portion 24c at its tip is erected on the inner face 29 of one insert halved object 2, a boss 36 with a tubular hole is erected on the inner face 39 of the other insert halved object 3, a stay 4 is clamped between the both insert halved objects, the projection portion 24c of the pin 24 penetrates an inner flange 38 provided in a tubular inner wall 37a of the tubular hole and is locked to the inner flange, and the both insert halved objects are integrated together. The inner flange 38 has a flange piece 38a having a thickness to lock and fix the projection portion 24c and being divided into a plurality of pieces by forming slits 38b, and a thin wall 38c having a thin film-like shape, connected to the circumferential edge of the respective flange pieces surrounding the slits in an slacked state and covering the slits.