A clock control apparatus configured to control a clock signal to be supplied to an electronic apparatus (100_1, 100_2) to which an external apparatus (201_1, 201_2) is connected, and which electronic apparatus performs at least one of inputting and outputting a signal to and from the external apparatus, the clock control apparatus comprising: a frequency detecting section configured to detect a frequency of said signal; a clock section which comprises a storing section configured to store a plurality of predetermined frequency values and rewritably store one or more frequency values other than the plurality of predetermined frequency values, a designating section configured to enable one of the frequency values stored in the storing section to be designated, and an oscillating section (1342, 1343) configured to output said clock signal having a frequency corresponding to the frequency value designated in the designating section; and a frequency setting section configured to determine if one of the stored predetermined frequency values should be designated to be the frequency of the clock signal based on the frequency detected by the frequency detecting section, and, if it is determined that one of the stored predetermined frequency values should be designated to be the frequency of the clock signal, to designate that frequency value so that the clock section outputs the clock signal having a frequency corresponding to that designated frequency value to be supplied to the electronic apparatus, wherein: the clock control apparatus further comprises a frequency notifying-storing section configured, if it is not determined that one of the stored predetermined frequency values should be designated to be the frequency of the clock signal, to transmit a request for designating a frequency value to the external apparatus via a communication line and to acquire an externally-designated frequency value based on a received response signal, wherein the frequency setting section is configured to store the externally-designated frequency value in the storing section and designate that frequency value so that the clock section outputs the clock signal having a frequency corresponding to that designated frequency value to be supplied to the electronic apparatus. a frequency detecting section configured to detect a frequency of said signal; a clock section which comprises a storing section configured to store a plurality of predetermined frequency values and rewritably store one or more frequency values other than the plurality of predetermined frequency values, a designating section configured to enable one of the frequency values stored in the storing section to be designated, and an oscillating section (1342, 1343) configured to output said clock signal having a frequency corresponding to the frequency value designated in the designating section; and a frequency setting section configured to determine if one of the stored predetermined frequency values should be designated to be the frequency of the clock signal based on the frequency detected by the frequency detecting section, and, if it is determined that one of the stored predetermined frequency values should be designated to be the frequency of the clock signal, to designate that frequency value so that the clock section outputs the clock signal having a frequency corresponding to that designated frequency value to be supplied to the electronic apparatus, wherein: the clock control apparatus further comprises a frequency notifying-storing section configured, if it is not determined that one of the stored predetermined frequency values should be designated to be the frequency of the clock signal, to transmit a request for designating a frequency value to the external apparatus via a communication line and to acquire an externally-designated frequency value based on a received response signal, wherein the frequency setting section is configured to store the externally-designated frequency value in the storing section and designate that frequency value so that the clock section outputs the clock signal having a frequency corresponding to that designated frequency value to be supplied to the electronic apparatus.