A monitoring system that includes a program execution apparatus and a server apparatus, the program execution apparatus comprising a plurality of modules (120, 131-133) including at least two monitoring modules, wherein each of the monitoring modules monitors at least another one of the plurality of monitoring modules as to whether the monitored monitoring module performs malicious operations; each monitoring module comprising: a holding unit for holding therein a piece of monitoring target information indicating at least another one of the plurality of monitoring modules as a monitoring target; a monitoring unit for monitoring the at least one module indicated by the piece of monitoring target information; a transmission unit for transmitting a result of the monitoring to the server apparatus; and an update unit for updating the held piece of monitoring target information with a new piece of monitoring target information for update acquired from the server apparatus; and the server apparatus comprising: an acquisition unit for acquiring the results of the monitoring performed by the respective monitoring modules; a specification unit (210,260) for specifying, based on all the acquired results of the monitoring, which of the plurality of monitoring modules performs malicious operations; and a transmission unit (210,260) for (i) holding therein pieces of monitoring target information that are the same as the pieces of monitoring target information held in the respective monitoring modules, (ii) restructuring the pieces of monitoring target information held therein such that each of the plurality of monitoring modules except the specified module is monitored by at least one of the non-specified monitoring modules, and (iii) transmitting, to any of the non-specified monitoring modules whose piece of monitoring target information needs to be updated as a result of the restructuring, one of the restructured pieces of monitoring target information corresponding to the any monitoring module. a holding unit for holding therein a piece of monitoring target information indicating at least another one of the plurality of monitoring modules as a monitoring target; a monitoring unit for monitoring the at least one module indicated by the piece of monitoring target information; a transmission unit for transmitting a result of the monitoring to the server apparatus; and an update unit for updating the held piece of monitoring target information with a new piece of monitoring target information for update acquired from the server apparatus; and the server apparatus comprising: an acquisition unit for acquiring the results of the monitoring performed by the respective monitoring modules; a specification unit (210,260) for specifying, based on all the acquired results of the monitoring, which of the plurality of monitoring modules performs malicious operations; and a transmission unit (210,260) for (i) holding therein pieces of monitoring target information that are the same as the pieces of monitoring target information held in the respective monitoring modules, (ii) restructuring the pieces of monitoring target information held therein such that each of the plurality of monitoring modules except the specified module is monitored by at least one of the non-specified monitoring modules, and (iii) transmitting, to any of the non-specified monitoring modules whose piece of monitoring target information needs to be updated as a result of the restructuring, one of the restructured pieces of monitoring target information corresponding to the any monitoring module. an acquisition unit for acquiring the results of the monitoring performed by the respective monitoring modules; a specification unit (210,260) for specifying, based on all the acquired results of the monitoring, which of the plurality of monitoring modules performs malicious operations; and a transmission unit (210,260) for (i) holding therein pieces of monitoring target information that are the same as the pieces of monitoring target information held in the respective monitoring modules, (ii) restructuring the pieces of monitoring target information held therein such that each of the plurality of monitoring modules except the specified module is monitored by at least one of the non-specified monitoring modules, and (iii) transmitting, to any of the non-specified monitoring modules whose piece of monitoring target information needs to be updated as a result of the restructuring, one of the restructured pieces of monitoring target information corresponding to the any monitoring module.