The purpose of the present disclosure is to provide a method for forming an in-mold coating multi-layer coating film, the method having a low environmental load and making it possible to form an in-mold coating multi-layer coating film having excellent interlayer adhesion between a first in-mold coating film and a second in-mold coating film. This method for forming an in-mold coating multi-layer coating film is configured as follows. The method for forming an in-mold coating multi-layer coating film includes: a step in which a specific first in-mold coating material composition (X) is injected between a molded base material and the inner wall of a first coating mold for a coating film, and an uncured first in-mold coating film is formed; a step in which the uncured first in-mold coating film is heated and cured; a step in which the first coating mold for a coating film is separated from the cured first in-mold coating film surface; a step in which a gap is provided between the cured first in-mold coating film surface and a second coating mold for a coating film; a step in which a specific second in-mold coating material composition (Y) is injected between the cured first in-mold coating film surface and the inner wall of the second coating mold for a coating film; and an uncured second in-mold coating film is formed; and a step in which the uncured second in-mold coating film is heated and cured.