A serial interface circuit, a semiconductor device, and a serial-parallel conversion method are provided. Conversion of write-in access to read-out access can be quickly performed in a serial memory.The disclosure is to generate first to nth timing signals respectively indicating timings that differ by 1 bit cycle of the bit string when receiving a serial signal including the bit string in a serial form and converting the bit string into a parallel form to obtain a parallel bit group. Each bit in the bit string is held at the timings of the first to tth timing signals as the standby bit group, the standby bit group is acquired at the timing of any one of the (t+1)th to nth timing signals as a part of the parallel bit group, and each bit in the bit string is held at the timings of the (t+1)th to nth timing signals and the held bit group is set as another part of the parallel bit group.