1. A substrate detection device, comprising: a converting unit configured to calculate a gray scale of a gray image of a circuit lead region of a substrate based on the gray image of the circuit lead region of the substrate, and to convert the gray image into an image of leads in the circuit lead region, based on a difference between the calculated gray scales of the leads and of the substrate in the circuit lead region; and a judging unit configured to judge a state of the leads in the circuit lead region based on the image of the leads in the circuit lead region.