Disclosed herein is a polyester resin-based toner binder which can impart both excellent anti-blocking property under high temperature and humidity conditions and low-temperature fixing properties to a toner. The toner binder for developing electrostatic images includes a polycondensation polyester resin produced in the presence of at least one titanium-containing catalyst (a) represented by the following general formula (I) or (II): Ti(—X)m(—OH)n  (I) O═Ti(—X)p(—OR)q  (II) wherein X is a residue formed by removing H of one OH group from a (poly)alkanolamine having 2 to 12 carbon atoms; R is H or a C1 to C8 alkyl group which may have 1 to 3 ether bonds; m is 1 to 4, n is 0 to 3, and the sum of m and n is 4; and p is 1 to 2, q is 0 to 1, and the sum of p and q is 2 (m, n, p, and q are each an integer).