A thin client system comprising: a virtual machine server on which one or more virtual machines operate; a thin client terminal to be connected to said virtual machines; a connection management server which manages connections between said thin client terminal and said virtual machines; and a virtual machine management server including a reverting processing execution unit which executes reverting processing in which a differential disk is deleted and said virtual machines are initialized, the differential disk represents a difference from a master image generated when said each virtual machine is executed in said thin client system which shares the master image and consumes a disk capacity; wherein said connection management server includes: a ranking information storage unit having a to-be-reverted list stored in which virtual machines to be reverted are arranged in descending order of differential disk capacities, a session management unit which determines said virtual machines to be allocated to said thin client terminal based on said to-be-reverted list, a ranking generation unit adapted to update the to-be-reverted list in response to an operation state of said session management unit, and a maintenance processing management unit which refers to said to-be-reverted list to request said virtual machine management server to execute reverting processing in descending order of differential disk capacities, wherein said session management unit, when available virtual machines include a virtual machine not included in said to-be-reverted list, allocates said virtual machine to said thin client terminal, and when all the available virtual machines are included in said to-be-reverted list, allocates said virtual machine whose differential disk capacity is the smallest in said to-be-reverted list to said thin client terminal, said ranking generation unit deletes said allocated virtual machine from said to-be-reverted list, said maintenance processing management unit refers to said to-be-reverted list and requests said virtual machine management server to execute the reverting processing of said virtual machines periodically. a virtual machine server on which one or more virtual machines operate; a thin client terminal to be connected to said virtual machines; a connection management server which manages connections between said thin client terminal and said virtual machines; and a virtual machine management server including a reverting processing execution unit which executes reverting processing in which a differential disk is deleted and said virtual machines are initialized, the differential disk represents a difference from a master image generated when said each virtual machine is executed in said thin client system which shares the master image and consumes a disk capacity; wherein said connection management server includes: a ranking information storage unit having a to-be-reverted list stored in which virtual machines to be reverted are arranged in descending order of differential disk capacities, a session management unit which determines said virtual machines to be allocated to said thin client terminal based on said to-be-reverted list, a ranking generation unit adapted to update the to-be-reverted list in response to an operation state of said session management unit, and a maintenance processing management unit which refers to said to-be-reverted list to request said virtual machine management server to execute reverting processing in descending order of differential disk capacities, wherein said session management unit, when available virtual machines include a virtual machine not included in said to-be-reverted list, allocates said virtual machine to said thin client terminal, and when all the available virtual machines are included in said to-be-reverted list, allocates said virtual machine whose differential disk capacity is the smallest in said to-be-reverted list to said thin client terminal, said ranking generation unit deletes said allocated virtual machine from said to-be-reverted list, said maintenance processing management unit refers to said to-be-reverted list and requests said virtual machine management server to execute the reverting processing of said virtual machines periodically. a ranking information storage unit having a to-be-reverted list stored in which virtual machines to be reverted are arranged in descending order of differential disk capacities, a session management unit which determines said virtual machines to be allocated to said thin client terminal based on said to-be-reverted list, a ranking generation unit adapted to update the to-be-reverted list in response to an operation state of said session management unit, and a maintenance processing management unit which refers to said to-be-reverted list to request said virtual machine management server to execute reverting processing in descending order of differential disk capacities, wherein said session management unit, when available virtual machines include a virtual machine not included in said to-be-reverted list, allocates said virtual machine to said thin client terminal, and when all the available virtual machines are included in said to-be-reverted list, allocates said virtual machine whose differential disk capacity is the smallest in said to-be-reverted list to said thin client terminal, said ranking generation unit deletes said allocated virtual machine from said to-be-reverted list, said maintenance processing management unit refers to said to-be-reverted list and requests said virtual machine management server to execute the reverting processing of said virtual machines periodically.