The invention provides a novel polyester adapted for use in a charge control agent for a toner for an electrophotographic process, having a charge stability, a high charge amount, an improved dispersibility and a biodegradability, and a producing method and an application technology therefore. According to the invention, there is provided polyhydroxyalkanoate comprising at least one unit represented by the following chemical formula in a molecule: --(O--CH(--(CH.sub.2).sub.m--CH.sub.2-p-C.sub.6H.sub.4--CONH-A.sub.1SO.su- b.2R.sub.1)CH.sub.2CO)-- (wherein R.sub.1 represents OH, a halogen atom, ONa, OK or OR.sub.1a; A.sub.1 and R.sub.1a each represents a substituted or unsubstituted aliphatic hydrocarbon structure, a substituted or unsubstituted aromatic cyclic structure or a substituted or unsubstituted heterocyclic structure; m represents an integer selected from a range of 0 to 7; and R.sub.1, A.sub.1, R.sub.1a and m have the aforementioned meanings independently for each unit when plural units are present), a charge control agent containing such polyhydroxyalkanoate, a toner utilizing such charge control agent, and an image forming method and an image forming apparatus utilizing such toner.