Disclosed is serial communication control system including: a first microcomputer; and a second microcomputer connected to the first microcomputer through a serial line, wherein the second microcomputer includes an R/B signal sending section to send one of a READY signal and a BUSY signal, the first microcomputer includes an R/B signal receiving section to receive the signal, and a first sending section to send first serial data to the second microcomputer when the R/B signal receiving section receives the READY signal, the second microcomputer includes a first receiving section to receive the first serial data, and a second sending section to send second serial data after the receiving operation of the first serial data by the first receiving section is completed and the R/B signal sending section thereafter sends the BUSY signal, and the first microcomputer includes a second receiving section to receive the second serial data.