1. An image forming device comprising: a plurality of image-forming units configured to form color image pieces different in color; a light-emitting element configured to emit light; a light-receiving element configured to receive light; a memory configured to store an execution condition for each color combination of two different colors, the execution condition being for correcting misregistration between the two different colors in two image pieces to be superposed one on the other; and a processor configured to: acquire a reference value set for the color combination of two different colors; read the execution condition from the memory; determine whether the reference value meets the execution condition, the determination being performed before forming a finished image including at least a color image portion formed by superposing one of the color image pieces on at least one of remaining color image pieces by using the plurality of image-forming units; form registration marks for the two different colors in response to determination that the reference value meets the execution condition; emit light toward a position of each of the registration marks by the light-emitting element; receive reflected light from the registration marks by the light-receiving element; and adjust image-forming positions corresponding to the two different colors subject to correction of misregistration based on outputs of the light-receiving element providing detection results of the position of each of the registration marks.