grooves 40 conversely a plurality of projections 41 each of the grooves 40 extends in the track direction indicated by the arrow tc the width of the groove 40 is not constant over the length thereof specifically in the embodiment shown in fig3 wherein the disk d rotates counterclockwise see the arrow n 1 each groove 40 has a greater width sa at its upstream end 42 a and a smaller width sb at its downstream end 42 b the groove 40 tapers regularly from the upstream end 42 a to the downstream end 42 b in other words the longitudinal side surfaces 43 a defining the groove 40 are nonparallel to each other but come closer to each other as proceeding from the upstream end 42 a to the downstream end 42 b the heat conductive layers 4 may be made by a process shown in fig4 a u02dc 4 e first as shown in fig4 a a copper base layer 49 a is formed on a dielectric substrate 3 a by sputtering or vapor deposition for example then as shown in fig4 b a first resist mask 69 a is formed on the copper base layer 49 a the mask 69 a may be made by forming a resist layer on the copper base layer 49 a and then subjecting the resist layer to light exposure and development after the mask 69 a is provided an additional copper layer 49 b is formed by plating for example on such portions of the copper base layer 49 a that are not covered by the mask 69 a.