A data transmitting circuit that converts parallel data into serial data to output the serial data, includes a first data input port that receives first parallel data at a first data rate based on a reference input clock; a second data input port that receives second parallel data at a second data rate lower than the reference input clock, a data expansion unit that generates expanded data by expanding a bit number of the second parallel data to a bit number of the first parallel data, a serial data generation unit that performs a process for generating first serial data by performing a serial conversion on the first parallel data based on the reference input clock and a process for generating second serial data by performing a serial conversion on the expanded data, and a data output port that outputs the first serial data or the second serial data.