We investigate the dependence of effective Q-factor on force sensitivity in Q-controlled phase-modulation atomic force microscopy. With Q-control, the phase noise density spectrum shows a characteristic dependence on modulation frequency (fm). The phase noise density spectrum is nearly constant in the low-fm region, whereas it decreases inverse-proportionally to fm in the high-fm region. Such a decrease enhances the force sensitivity. We demonstrate that force sensitivity can be markedly increased with Q-control to exceed the limit of force sensitivity without Q-control.