An extremely thin copper foil is provided that enables formation of highly fine different wiring patterns with a line/space (L/S) of 10 μm or less/10 μm or less on two sides of the copper foil and is thus usable as an inexpensive and readily processable substitution for silicon and glass interposers. The extremely thin copper foil includes, in sequence, a first extremely thin copper layer, an etching stopper layer, and the second extremely thin copper layer. Two sides of the extremely thin copper foil each have an arithmetic average roughness Ra of 20 nm or less.