PURPOSE: To obtain an acetate fiber having softness and considerable bulky feeling.

CONSTITUTION: This acetate fiber has a side-by-side conjugate structure consisting of a cellulose or a cellulose acetate having an acetylation degree of <48.8% and a cellulose triacetate having an acetylation degree of 56.2-62.5%. The areal ratio of the constituent component on the cross-section varies in the direction of the fiber axis and the fiber has a bulkiness of ≥2.0cm3/g. It can be produced by the alkali treatment of a precursor fiber having a side-by- side conjugate structure consisting of a cellulose diacetate having an acetylation degree of <56.2% and a cellulose triacetate having an acetylation degree of ≥56.2% and having an areal ratio of the constituent component on the cross-section varying in the direction of fiber axis.