A safety instrument system comprising: a plurality of positioners for controlling the degrees of opening of a plurality of emergency cutoff valves that are provided in pipes in a plant; and a higher-level system for controlling partial stroke tests, PSTs, of the emergency cutoff valves, wherein: each positioner comprises: a PST executing portion for executing a PST in the emergency cutoff valve in response to an initiating instruction from the higher-level system or an initiating instruction from an inputting device at the site wherein the positioner is provided; a PST execution status acquiring portion for acquiring PST execution status information for another positioner; a PST initiation prohibition evaluating portion (21, 21 a) for evaluating whether or not a PST initiation is permitted, based on a PST execution status of another positioner, when a PST initiating instruction has been received from the higher-level system or from the inputting device. a plurality of positioners for controlling the degrees of opening of a plurality of emergency cutoff valves that are provided in pipes in a plant; and a higher-level system for controlling partial stroke tests, PSTs, of the emergency cutoff valves, wherein: each positioner comprises: a PST executing portion for executing a PST in the emergency cutoff valve in response to an initiating instruction from the higher-level system or an initiating instruction from an inputting device at the site wherein the positioner is provided; a PST execution status acquiring portion for acquiring PST execution status information for another positioner; a PST initiation prohibition evaluating portion (21, 21 a) for evaluating whether or not a PST initiation is permitted, based on a PST execution status of another positioner, when a PST initiating instruction has been received from the higher-level system or from the inputting device. each positioner comprises: a PST executing portion for executing a PST in the emergency cutoff valve in response to an initiating instruction from the higher-level system or an initiating instruction from an inputting device at the site wherein the positioner is provided; a PST execution status acquiring portion for acquiring PST execution status information for another positioner; a PST initiation prohibition evaluating portion (21, 21 a) for evaluating whether or not a PST initiation is permitted, based on a PST execution status of another positioner, when a PST initiating instruction has been received from the higher-level system or from the inputting device. a PST executing portion for executing a PST in the emergency cutoff valve in response to an initiating instruction from the higher-level system or an initiating instruction from an inputting device at the site wherein the positioner is provided; a PST execution status acquiring portion for acquiring PST execution status information for another positioner; a PST initiation prohibition evaluating portion (21, 21 a) for evaluating whether or not a PST initiation is permitted, based on a PST execution status of another positioner, when a PST initiating instruction has been received from the higher-level system or from the inputting device.