Abstract: There is provided a controller for an image forming system including a head in which a plurality of nozzles is formed. The controller includes: a first communication interface configured to communicate with a line sensor including imaging elements arranged at a predetermined interval and having an element-lacking-part at which an interval between two elements is larger than the predetermined interval; and a second communication interface. The controller is configured to: form a test pattern including patterns; obtain a test pattern data generated by imaging the test pattern with the line sensor; correct a position of the patterns on the test pattern data based on a position of the element-lacking-part; and detect a discharge-defect-nozzle based on the corrected test pattern data.