An interface conversion device that enables the communication between a first device having a first interface and a second device having a second interface with a data transmission speed faster than that of the first interface comprising: a first transmission/reception part connected to the first interface; a second transmission/reception part connected to the second interface; a data storage part connected to the second transmission/reception part, and storing the data transmitted from the second transmission/reception part; an information conversion part connected to the first transmission/reception part and the data storage part, and at least converting the data transmitted from the second interface stored in the data storage part so as to fit the first interface; a first information acquisition part connected to the first transmission/reception part, and acquiring the information related to the data transmission speed of the first interface; and a transmission speed management part determining the optimum data transmission speed related to the second interface based on the information acquired by the first information acquisition part, wherein the transmission speed management part determines a first reference speed based on the type and version information of the communication standard related to the first interface acquired by the first information acquisition part, characterized in that the transmission speed management part determines the actual data transmission speed of the first interface as a second reference speed, and determines the optimum speed in the range of an absolute value for the first reference speed ± α, that is the first reference speed -α ≦ the second reference speed ≦ the first reference speed + α, the α is a rate constant that enables the communication between the first interface and the second interface without the data transmission standby time of the second transmission/reception part exceeding 1 ms, the α is determined by the communication state between the first interface and the second interface, the specifications of the first interface and the second interface, and the like. a first transmission/reception part connected to the first interface; a second transmission/reception part connected to the second interface; a data storage part connected to the second transmission/reception part, and storing the data transmitted from the second transmission/reception part; an information conversion part connected to the first transmission/reception part and the data storage part, and at least converting the data transmitted from the second interface stored in the data storage part so as to fit the first interface; a first information acquisition part connected to the first transmission/reception part, and acquiring the information related to the data transmission speed of the first interface; and a transmission speed management part determining the optimum data transmission speed related to the second interface based on the information acquired by the first information acquisition part, wherein the transmission speed management part determines a first reference speed based on the type and version information of the communication standard related to the first interface acquired by the first information acquisition part, characterized in that the transmission speed management part determines the actual data transmission speed of the first interface as a second reference speed, and determines the optimum speed in the range of an absolute value for the first reference speed ± α, that is the first reference speed -α ≦ the second reference speed ≦ the first reference speed + α, the α is a rate constant that enables the communication between the first interface and the second interface without the data transmission standby time of the second transmission/reception part exceeding 1 ms, the α is determined by the communication state between the first interface and the second interface, the specifications of the first interface and the second interface, and the like.