A machine tool system comprising: a plurality of machine tools; and a plurality of mobile robots configured to convey workpieces to the plurality of machine tools; wherein each of the plurality of machine tools comprises a machine tool control unit configured to communicate with the plurality of mobile robots and to issue work requests for the individual machine tools; and wherein each of the plurality of mobile robots comprises a mobile robot control unit configured to determine possible work start times for said mobile robot based on the work requests for the individual machine tools sent out from the machine tool control unit; and wherein the machine tool system further comprises a determining unit configured to compare the possible work start times respectively calculated by the mobile robots to determine the mobile robot with the earliest possible work start time, and to return information to the mobile robots to allow a determination of a mobile robot in charge of the work, the mobile robot in charge of the work being configured to incorporate that work into its work schedule, wherein the work requests include a workpiece conveyance work request and a maintenance work request, wherein the workpiece conveyance work request is either a request for attachment of a workpiece to a machine tool or a request for unloading the workpiece from the machine tool, wherein the maintenance work request is a request for any one of work of removing chips from the machine tool, conveying lubricants and supplying lubricants to an operating part of the machine tool, and conveying a tool and inserting the tool in a tool storage of the machine tool, wherein the mobile robots each have a manipulator having three axes or more of freedom on an unmanned cart and a work storage configured to store movement routes or work procedures which have been programmed into the work storage or taught to the mobile robot by an operator in advance, wherein the mobile robot control unit is configured to store required times taken for the work procedures stored in the respective work storages of the plurality of mobile robots and to calculate the possible work start times of the mobile robots based on the required times, wherein the determining unit is configured to send information to the mobile robots to allow a mobile robot to judge, based on the possible work start times of the plurality of the mobile robots, whether it is in charge of the work, and wherein the mobile robot control unit is further configured to calculate the possible work start times of the mobile robots based on a priority order assigned in advance to each of the workpiece conveyance work, the tool conveyance work, the chip removal work, and the lubricant conveyance work. a plurality of machine tools; and a plurality of mobile robots configured to convey workpieces to the plurality of machine tools; wherein each of the plurality of machine tools comprises a machine tool control unit configured to communicate with the plurality of mobile robots and to issue work requests for the individual machine tools; and wherein each of the plurality of mobile robots comprises a mobile robot control unit configured to determine possible work start times for said mobile robot based on the work requests for the individual machine tools sent out from the machine tool control unit; and wherein the machine tool system further comprises a determining unit configured to compare the possible work start times respectively calculated by the mobile robots to determine the mobile robot with the earliest possible work start time, and to return information to the mobile robots to allow a determination of a mobile robot in charge of the work, the mobile robot in charge of the work being configured to incorporate that work into its work schedule, wherein the work requests include a workpiece conveyance work request and a maintenance work request, wherein the workpiece conveyance work request is either a request for attachment of a workpiece to a machine tool or a request for unloading the workpiece from the machine tool, wherein the maintenance work request is a request for any one of work of removing chips from the machine tool, conveying lubricants and supplying lubricants to an operating part of the machine tool, and conveying a tool and inserting the tool in a tool storage of the machine tool, wherein the mobile robots each have a manipulator having three axes or more of freedom on an unmanned cart and a work storage configured to store movement routes or work procedures which have been programmed into the work storage or taught to the mobile robot by an operator in advance, wherein the mobile robot control unit is configured to store required times taken for the work procedures stored in the respective work storages of the plurality of mobile robots and to calculate the possible work start times of the mobile robots based on the required times, wherein the determining unit is configured to send information to the mobile robots to allow a mobile robot to judge, based on the possible work start times of the plurality of the mobile robots, whether it is in charge of the work, and wherein the mobile robot control unit is further configured to calculate the possible work start times of the mobile robots based on a priority order assigned in advance to each of the workpiece conveyance work, the tool conveyance work, the chip removal work, and the lubricant conveyance work.