A method includes calculating a size of a buffer included in a client device by multiplying a band by a round trip time, calculating an integer value by turning a real number into an integer, and setting the integer value as an unused size, the real number being obtained by dividing the size by a transmission data size, setting a value obtained by dividing the round trip time by the integer value or a value obtained by dividing the transmission data size by the band as a transmission interval, generating image data of a screen of a remote desktop and transmitting the image data to the client device when the unused size is greater than the first value, and subtracting a second value from the unused size, and every time a response to the image data is received, adding the second value to the unused size.