The present invention provides a serial/parallel conversion circuit that has both a serial/parallel conversion function and a buffer function for absorbing clock frequency differences, together with a data transfer control device and electronic equipment. The serial/parallel conversion circuit (elasticity buffer) comprises a data holding register which holds serial data DIN that is input based on a CLK1 clock (480 MHz) in USB 2.0 HS mode; a determination circuit which determines whether or not held data is valid, by unit of a data cell; and a selector which outputs from the data holding register the data of data cells that have been determined to be valid, based on a CLK2 clock (60 MHz) having a frequency lower than that of CLK1. A data cell in which data of the first bit has been determined to be valid is deemed to be valid in the next CLK2 clock cycle. The determination of whether or not data cells are valid is done in each clock cycle of CLK2, and the output of data in a data cell that is determined not to be valid is made to wait for one clockcycle. Awrite pulse signal is generated and the data holding register and data status register are operated thereby.