A voltage conversion device and a voltage conversion method are provided in which, even immediately after switching a switching frequency, it is possible to suppress fluctuation of output voltage and possible to output a constant voltage in a stable manner. When switching the switching frequency from a first frequency to a second frequency, a duty ratio is changed in a first cycle of a PWM signal immediately after switching so as to be smaller than the duty ratio before switching. The amount of change in this case is set such that a lower limit value of inductor current immediately after switching the switching frequency matches the lower limit value in a steady state. With this sort of change, an increase in the inductor current immediately after switching is suppressed, fluctuation of the output voltage is suppressed, and a stable constant voltage is outputted to a load.