The present invention provides a test dummy wafer used in the process for manufacturing a semiconductor device, which has more excellent etching resistance than a silicon wafer, and excellent mirror surface properties and evenness required for a substrate, and which causes no contamination source in the manufacturing process. The dummy wafer is composed of glassy carbon, and at least one side thereof is preferably polished to a mirror surface having a surface roughness Ra of not more than 0.005 .mu.m. The dummy wafer of the present invention has excellent characteristics as a dummy wafer for monitoring the thickness of a CVD film. The dummy wafer having specific electric resistance of not more than 0.1 .OMEGA..multidot.cm exhibits excellent characteristics as a dummy wafer for monitoring the thickness of a film formed by sputtering and confirming cleanliness.