A heat treatment apparatus for heat-treating powder particles each containing a binder resin and a colorant includes: a powder particle supply unit (3); a hot air supply unit (2); a cold air supply unit (4); a regulating unit (6); and a collection unit (5). The apparatus is characterized in that: a protrusion with a height of 2 mm or more and 50 mm or less is provided in a region on a downstream side of the powder particle supply unit and on an upstream side of the cold air supply unit in an inner wall surface of the treatment chamber; and the heat treatment apparatus has a cross-section plane, which is perpendicular to a center axis of the treatment chamber and situated at the region where the protrusion is provided, and Dmin and Dmax satisfy the following relation 0.50<=Dmin/Dmax<1.0 where, Dmin represents a minimum value of the distance of a gap between the treatment chamber and the columnar member measured in the cross-section plane, and Dmax represents a maximum value of the distance of a gap between the treatment chamber and the columnar member measured in the cross-section plane.