An image forming apparatus comprising: image bearing member (101, 201) on which an image is formed; a charging means (106, 202) configured to charge the image bearing member (101, 201); transfer means (108, 204) configured to transfer the image formed on the image bearing member (101, 201) onto a transfer member; voltage application means (205, 206, 301, 401) configured to apply a voltage to the charging means (106, 202) and a predetermined voltage to the transfer means (108, 204); current detection means configured to detect a current flowing to the image bearing member (101, 201) via the transfer means (108, 204) when a voltage is applied to the transfer means (108, 204); and control means configured to control operations of the image forming apparatus, such that in a state where the voltage is applied to the charging means (106, 202), a surface potential of the image bearing member (101, 201) is determined using a first voltage applied from the voltage application means (205, 206, 301, 401) when a current value obtained by, after applying the predetermined voltage to the transfer means (108, 204), detecting the current value with the current detection means while changing the predetermined voltage to a positive direction, reaches a discharge current value, and a second voltage applied from the voltage application means (205, 206, 301, 401) when a current value obtained by, after applying the predetermined voltage to the transfer means (108, 204), detecting the current value with the current detection means while changing the predetermined voltage to a negative direction, reaches the discharge current value. image bearing member (101, 201) on which an image is formed; a charging means (106, 202) configured to charge the image bearing member (101, 201); transfer means (108, 204) configured to transfer the image formed on the image bearing member (101, 201) onto a transfer member; voltage application means (205, 206, 301, 401) configured to apply a voltage to the charging means (106, 202) and a predetermined voltage to the transfer means (108, 204); current detection means configured to detect a current flowing to the image bearing member (101, 201) via the transfer means (108, 204) when a voltage is applied to the transfer means (108, 204); and control means configured to control operations of the image forming apparatus, such that in a state where the voltage is applied to the charging means (106, 202), a surface potential of the image bearing member (101, 201) is determined using a first voltage applied from the voltage application means (205, 206, 301, 401) when a current value obtained by, after applying the predetermined voltage to the transfer means (108, 204), detecting the current value with the current detection means while changing the predetermined voltage to a positive direction, reaches a discharge current value, and a second voltage applied from the voltage application means (205, 206, 301, 401) when a current value obtained by, after applying the predetermined voltage to the transfer means (108, 204), detecting the current value with the current detection means while changing the predetermined voltage to a negative direction, reaches the discharge current value.