The broadening effect in photoelectron angular distributions (PADs) observed by Freeman et al. Is studied theoretically. Using a nonperturbative scattering theory developed for multiphoton ionization with the inclusion of spontaneous emission, we calculate the PADs for above-threshold ionization (ATI) peaks. The numerical calculations from our theory reproduce the kinetic-energy dependence and the laser-intensity dependence of PADs of ATI peaks observed by Freeman et al., [Phys. Rev. Lett. 57 (1986) 3156] and provide an evidence for the existence of the ponderomotive momentum of intense laser fields.