To provide a method for producing a carbon fiber sheet, by which the stable carbon fiber substrate sheet can be obtained without cutting a thin sheet having good flat surface smoothness.

This method for producing the carbon fiber sheet is characterized by continuously thermally treating a thermal treatment raw carbon fiber sheet or thermal treatment raw oxidized fiber sheet 2 containing a resin carbonizable on the thermal treatment at 500 to 1,000°C at a contact pressure of 4.9 to 98 kPa, and in a relaxed state of 3 to 10 % in a low treatment oven 4 in an inactive gas atmosphere, and then thermally treating the treated sheet at 1,200 to 2,500°C at a contact pressure of ≤4.9 kPa and in a relaxed state of 0 to 3 % in a high temperature treatment oven 20. It is preferable to use polyacrylonitrile-based fibers for the raw sheet.