PURPOSE: To form a CVD diamond layer having substantially uniform thickness on a stationary base body.

CONSTITUTION: To partially decompose a gaseous hydrocarbon mixture for the purpose of formation of the CVD diamond layer on the base body, the gaseous hydrocarbon mixture is run on a thin and long hot filament 18 and the diamond layer is chemically vapor-deposited(CVD) on the base body held at a high CVD diamond formation temperature. At this time, the base body has a maximum diameter R defined as the maximum distance from its center to the outermost surface coated with the CVD diamond layer and the base body is arranged at a distance C from the filament in parallel to the filament about the axis of the base body. As R/C decreases more and more, the thickness of the CVD diamond layer becomes more and more uniform.