A method of inspecting a RAM of a programmable logic controller is provided that enables all unit storage regions in a RAM inspection region to be inspected in about several minutes and that meets a predetermined safety standard. The inspection method includes the steps of: (a) dividing a RAM inspection region, which is a region of the RAM to be inspected, into a plurality of blocks; and (b) by a control device, writing a predetermined value into unit storage regions in a combination of blocks obtained by combining two blocks extracted from the divided blocks and thereafter sequentially inspecting whether or not values read from the unit storage regions each coincide with the written value. Step (b) is executed for all combinations of two blocks extracted from the divided blocks.