A false act of a clerk at a store is monitored at a host computer under utilization of a communication network by detecting a false key operation in reference to a count which can be flexibly set in an item sales register or the like. The sales processing unit reads out in sequence the stored function numbers, collates them with the function numbers of the depressing limit in sequence, and if the result shows a coincided state, the sales processing unit adds 1 to the limit counter of the corresponding key, compares the value of the limit counter of the corresponding key with the corresponding limit count of the depressing limit master, and if the value of the limit counter exceeds the limit count, the sales processing unit outputs to the communication unit an instruction that the fact showing of the depressing action of the corresponding key exceeded the limit count is sent to the host computer, and causes the communication unit to send it. In addition, an instruction for displaying an alarm at the screen display unit indicating that the depressing of the corresponding key exceeds the limit count is issued and this is displayed at the display.