A parallel-to-serial conversion circuit for converting pieces of parallel data into serial data, and a parallel-to-serial converting method thereof include: a shifter configured to sequentially shift an initiation signal to generate a plurality of transfer activation signals; a valid duration generator configured to define valid durations of the plurality of pieces of parallel data based on a clock and the plurality of transfer activation signals; and an output unit configured to receive the plurality of pieces of parallel data whose valid duration has been defined and to drive an output in response to a piece of data from among the received parallel data whose valid duration has begun.