An instrumentation system, comprising: a plurality of field devices, each of the field devices being configured to perform at least one of measuring and operation of a target; a plurality of I/O modules, the field devices being connectable to each of the I/O modules; a connection device comprising a plurality of slots for allowing the I/O modules to be attached to the plurality of slots, respectively; and a host control device electrically connected to the connection device and configured to individually set the I/O modules attached to the slots to have a redundant configuration or a single configuration, the host control device making two I/O modules set to have the redundant configuration, among the I/O modules attached to the slots, be paired, and operating one of the two I/O modules under a condition where one of the two I/O modules is in an activated state and the other of the two I/O modules is in a standby state, the instrumentation system being characterized by: the host control device operating a first I/O module set to have the single configuration, among the I/O modules attached to the slots, in a pseudo redundant configuration, the pseudo redundant configuration being a condition where the first I/O module is operated in the activated state, in a state in which an I/O module to be paired with the first I/O module is not decided, wherein when the first I/O module is required to be replaced, the host control device is configured to make the first I/O module and a second I/O module newly attached to one of the slots redundant. a plurality of field devices, each of the field devices being configured to perform at least one of measuring and operation of a target; a plurality of I/O modules, the field devices being connectable to each of the I/O modules; a connection device comprising a plurality of slots for allowing the I/O modules to be attached to the plurality of slots, respectively; and a host control device electrically connected to the connection device and configured to individually set the I/O modules attached to the slots to have a redundant configuration or a single configuration, the host control device making two I/O modules set to have the redundant configuration, among the I/O modules attached to the slots, be paired, and operating one of the two I/O modules under a condition where one of the two I/O modules is in an activated state and the other of the two I/O modules is in a standby state, the instrumentation system being characterized by: the host control device operating a first I/O module set to have the single configuration, among the I/O modules attached to the slots, in a pseudo redundant configuration, the pseudo redundant configuration being a condition where the first I/O module is operated in the activated state, in a state in which an I/O module to be paired with the first I/O module is not decided, the host control device operating a first I/O module set to have the single configuration, among the I/O modules attached to the slots, in a pseudo redundant configuration, the pseudo redundant configuration being a condition where the first I/O module is operated in the activated state, in a state in which an I/O module to be paired with the first I/O module is not decided, wherein when the first I/O module is required to be replaced, the host control device is configured to make the first I/O module and a second I/O module newly attached to one of the slots redundant.