Image forming apparatus and detecting method of pattern image regarding image quality adjustment

An image forming apparatus includes: an obtaining unit that radiates light from a light-emitting unit onto the image carrier and that obtains an output value of a light-receiving unit having received the light reflected from the image carrier; a light amount setting unit that calculates a required light amount value and that sets a light amount adjusting value; a detecting unit that detects a pattern image on the image carrier and by judging whether an output value obtained by radiating light of the light amount adjusting value exceeds a predetermined threshold; and a controller that controls image formation on the image carrier based on a detected result, wherein the detecting unit reduces the threshold when the required light amount value exceeds the limiting value depending on degree of the excess.