To assure the exquisiteness of discoloration and safety and to prevent environmental pollution by disposing a reversibly thermally discolorable layer which discolors with a temp. change on a resin sheet base material and using a polyolefin resin which does not contain a chlorine compd. and has the adaptability to high-frequency welding as the resin sheet base material.

A surface of a transparent polyolefin resin sheet base material 2 is provided with a reversibly thermally discolorable layer 3 by printing the base material with solid patterns of a star shape with the thermally discolorable ink which is prepd. by dispersing microcapsule pigments enclosing a reversibly thermally discolorable compsn. into a resin soln. prepd. by dissolving an acrylic resin into cyclohexane. A thermally discolorable toy 1 is obtd. by placing a similar white polyolefin resin sheet on the upper side of the reversibly thermally discolorable layer 3, subjecting the sheet to high-frequency welding along the ends and simultaneously providing the sheet with an air blowing-in port. This thermally discolorable toy 1 may be played like a balloon by blowing the air into the toy. In addition, the blue star shapes are visible on the white ground at room temp. and the thermally discolorable toy is usable as a toy which turns white as the temp. rises.