Image forming apparatus capable of improving adjustment accuracy of toner image forming condition, and image density adjustment method

An image forming apparatus, comprising:

• a toner image forming portion including at least a developing member which develops an electrostatic latent image formed on an image-carrying member using toner and configured to form a toner image;
• and
• a processor, wherein the processor functions as:
• a first acquisition processing portion which acquires a toner concentration of a predetermined specific toner image that is formed by the toner image forming portion;
• a second acquisition processing portion which acquires a first toner image forming condition of the toner image forming portion;
• and
• a third acquisition processing portion which acquires, based on the first toner image forming condition acquired by the second acquisition processing portion, a relational expression configured for calculating a second toner image forming condition of the toner image forming portion by substituting the toner concentration into the relational expression, wherein
• the third acquisition processing portion uses specific expressions that respectively correspond to a plurality of coefficients included in the relational expression to calculate the plurality of coefficients corresponding to the first toner image forming condition acquired by the second acquisition processing portion, the specific expressions each indicating a relationship between one of the plurality of coefficients and the first toner image forming condition.