1. An image processing method for performing image processing on a captured image by using a control device, an image processing device, and an imaging device that are capable of communication with each other, the method comprising: acquiring a main captured image by capturing, by using the imaging device, an image of an object by using a predetermined main imaging parameter, and performing, by using the image processing device, main image processing on the main captured image; transmitting, to the control device, a result of the main image processing performed on the main captured image; acquiring a sub-captured image by capturing, by using the imaging device, an image of the object by using a sub-imaging parameter, and performing, by using the image processing device, sub-image processing on the sub-captured image; and comparing the result of the main image processing performed on the main captured image with a result of the sub-image processing performed on the sub-captured image and, if the result of the sub-image processing falls within a predetermined processing result criteria range, performing updating by setting the sub-imaging parameter as a new main imaging parameter, and if the result of the sub-image processing does not fall within the predetermined processing result criteria range, refraining from updating.