1. An information processing system comprising: a processor and a memory, the processor and the memory cooperating to function as: a job management unit configured to manage a job consisting of multiple tasks that is generated in accordance with a user request; and multiple back-end units configured to acquire tasks from the job management unit if a processing standby status exists and carry out specific task processing, wherein the job management unit further comprises: a processing sequence information storage unit configured to store processing sequence information that indicates processing sequence in which respective tasks included in the job are processed; a task information storage unit configured to store task information which includes processing times corresponding to processing contents of the respective tasks and notification time intervals that are time intervals at which each back-end unit notifies the job management unit that a task processing is in progress; and a job information storage unit configured to store job information including information including a task that is foremost in processing sequence among multiple tasks included in the job, a status of the task, and a final notification time at which a notification is received from the back-end unit in the process of executing the task that task processing is in progress; wherein the back-end units periodically notify the job management unit that task processing is in progress, and wherein the job management unit which receives a task request from the back-end unit: discriminates whether a status of a task, which matches the task for which the task request was issued and which is currently foremost in processing sequence, indicates that task processing is in progress based on job information stored in the job information storage unit; discriminates whether a prescribed time has elapsed from the final notification time in the case where the status of the task, which matches the task for which the task request was issued and which is currently foremost in processing sequence, indicates that task processing is in progress; issues a command to suspend task processing to the back-end unit that has not completed the task processing and issues a command to alternatively execute the task processing in the process of execution to the back-end unit that issued the task request in the case where the prescribed time has elapsed from the final notification time.