To obtain a polyfumarate having carboxyl groups which excels in alkali solubility and heat resistance, to provide its manufacturing method, and to obtain an alkaline base resin for a photosensitive resin composition using the above polyfumarate.

The polyfumarate having carboxyl groups is composed of a repeating constituting unit derived from a fumaric diester and a constituting unit derived from a substituted ethylene having a carboxyl group. Furthermore, the polyfumarate is a compound having at least one constituting unit based on the fumaric diester and at least one constituting unit based on a carboxyl group-containing monomer and has a molecular weight of 1,000-300,000, and simultaneously, an acid value of 50-250 mgKOH/g.