To provide a method for changing the color of hair bundle with a treatment agent in a short time with low energy consumption.

The method comprises the following steps: first, a protective film 7 is placed on a flat plate-like hair bundle placing part 4; secondly, the hair bundle 2 is put thereon and coated and spread with the treatment agent 3; thirdly, an excess portion of the protective film 7 is folded and put on the hair bundle 2; fourthly, a flat plate-like hot plate 6 is applied to the hair bundle 2 for a fixed time; finally, the treatment agent 3 is rinsed off from the hair bundle 2, and as necessary, the resultant hair bundle 2 is neutralized.