An information processing system comprising: a Web server which determines whether a session identifier is added to a received request, and in a case where a session identifier is not added to the received request, generates a session identifier when authentication succeeds and in a case where the session identifier is added to the received request, performs processing according to the received request; and an information processing apparatus capable of communicating with the Web server, which includes a Web browser for performing control to display an operation screen on a display unit using screen information provided by the Web server, and a service provider for performing control to execute a job according to an instruction from the Web server, the Web server comprising: generation means for generating, when the authentication using authentication information received from the Web browser succeeds, a session identifier to be used for communication with the Web browser; means for transmitting the screen information and the session identifier generated by the generation means to the Web browser; means for receiving from the Web browser an operation content on the operation screen based on the screen information; means for transmitting, upon receiving from the Web browser the operation content on the operation screen based on the screen information, an instruction for executing a job corresponding to the operation content and instruction information to the service provider, the instruction information instructing the service provider to use, as a session identifier for communication with the Web server, the session identifier generated by the generation means; and reception means for receiving, from the service provider, completion information indicating an execution result of the job executed according to the instruction and the session identifier, the Web browser of the information processing apparatus comprising: first management means for managing a session identifier received from the Web server; means for displaying the operation screen on the display unit using the screen information received from the Web server; and means for transmitting, to the Web server, the operation content on the operation screen together with the session identifier managed by the first management means, and the service provider of the information processing apparatus comprising: means for executing, upon receiving the instruction from the Web server, the job according to the instruction; and second management means for managing a session identifier which is indicated by the instruction information received from the Web server; transmission means for transmitting, upon completion of the job, to the Web server without intervening the Web browser, completion information indicating an execution result of the job together with the session identifier managed by the second management means in accordance with the instruction information. generation means for generating, when the authentication using authentication information received from the Web browser succeeds, a session identifier to be used for communication with the Web browser; means for transmitting the screen information and the session identifier generated by the generation means to the Web browser; means for receiving from the Web browser an operation content on the operation screen based on the screen information; means for transmitting, upon receiving from the Web browser the operation content on the operation screen based on the screen information, an instruction for executing a job corresponding to the operation content and instruction information to the service provider, the instruction information instructing the service provider to use, as a session identifier for communication with the Web server, the session identifier generated by the generation means; and reception means for receiving, from the service provider, completion information indicating an execution result of the job executed according to the instruction and the session identifier, first management means for managing a session identifier received from the Web server; means for displaying the operation screen on the display unit using the screen information received from the Web server; and means for transmitting, to the Web server, the operation content on the operation screen together with the session identifier managed by the first management means, and means for executing, upon receiving the instruction from the Web server, the job according to the instruction; and second management means for managing a session identifier which is indicated by the instruction information received from the Web server; transmission means for transmitting, upon completion of the job, to the Web server without intervening the Web browser, completion information indicating an execution result of the job together with the session identifier managed by the second management means in accordance with the instruction information.