An A/D converter that is attached to a programmable controller (PLC) and sequentially converts an analog value inputted from outside into a digital value. The A/D converter includes: a shared memory that can read-access from a CPU unit that controls the entire PLC and includes a log storage area with a ring buffer configuration for sequentially logging the digital value and a parameter storage area for storing a head pointer serving as a parameter indicating a position where a next log data is stored; and a logging executing unit that writes a digital value in an address indicated by the head pointer in the log storage area as log data and updates the head pointer.