Adhesive composition, adhesive, adhesive sheet, and double-sided adhesive sheet

Provided is an adhesive composition which uses a recycled material that is environmentally friendly to the earth, has good adhesive properties with respect to various adherends, and has excellent adhesive properties such as adhesive force and holding power, and which is a polyester resin composition containing a polyester resin (A), the polyester resin (A) contains a structural unit derived from at least one compound (a1) selected from a dimer acid and a dimer glycol, and a structural unit derived from an aromatic compound (a2), and the ratio (X1/X2) of the molar concentration (X1) of the structural unit derived from the compound (a1) to the molar concentration (X2) of the structural unit derived from the aromatic compound (a2) is greater than 1.0. 作为使用对地球环境友好的再生材料、对各种被粘物的粘合物性良好、粘合力和保持力等粘合物性优异的粘合剂组合物，提供一种粘合剂组合物，其为含有聚酯系树脂(A)的聚酯系树脂组合物，聚酯系树脂(A)含有：源自二聚酸类和二聚二醇中的至少1种化合物(a1)的结构单元、和源自芳香族化合物(a2)的结构单元，源自上述化合物(a1)的结构单元的摩尔浓度(X1)相对于源自上述芳香族化合物(a2)的结构单元的摩尔浓度(X2)的比例(X1/X2)大于1.0。