A signal tracing control system provided in a programmable machine controller (PMC) which controls a machine in response to a signal from a numerical control apparatus. A signal of a monitored RAM (1) is monitored for a change by a monitoring circuit (21). When a signal change is detected by the signal monitoring circuit (21), it applies a signal to a writing control circuit (22) which then writes the data of the monitored RAM (1) into a tracing buffer (2). This cycle of operation is repeated. Thus, only when a data item in the monitored RAM (1) is changed, is the signal traced. Effective signal tracing can therefore be effected with a small memory capacity.