To provide a thin film forming method for forming a film excellent in flatness on a body to be printed without being influenced by minute projected parts formed on a letterpress, and a thin film forming device.

In the thin film forming method employing a thin film forming device equipped with a letterpress 5, on the surface of which a large number of minute projected parts are formed in order to print a predetermined pattern on a body 9 to be printed, a printing stage 4, onto which the body to be printed is fixed, and a printing roll 6, onto which the letterpress is installed, printing liquid filled on and between the minute projected parts 7 on the letterpress 5 is transferred on the body to be printed. In the transfer of the liquid in the beginning turning of the printing roll and that in its next turning, the relative position between is shifted by a distance x satisfying an expression N<(x/p) cosθ<N+1 (wherein 0°≤θ<90°, x is the shifted distance of the minute projected part, p is the pitch between the minute projected parts, N is an integer, θ is an angle made between the rotary axis of the printing roll and the arrangement of the minute projected parts on the surface of the letterpress) so as to transfer the printing liquid again from the letterpress to the same body to be printed.