An image forming apparatus comprising: an image forming portion (PY, PM, PC, PK) configured to form a toner image; an endless belt which is provided opposed to said image forming portion (PY, PM, PC, PK) and onto which the toner image formed on said image forming portion (PY, PM, PC, PK) is transferred; a primary transfer roller (15Y, 15M, 15C, 15K) provided opposed to said image forming portion (PY, PM, PC, PK) through said endless belt and configured to form a primary transfer portion (T1Y, T1M, T1C, T1K) for transferring the toner image formed on said image forming portion (PY, PM, PC, PK); a secondary transfer roller contacting an outer surface of said endless belt and configured to transfer the toner image from said endless belt onto a recording material at a secondary transfer portion; a tiltable steering roller configured to adjust a position of said endless belt with respect to a widthwise direction, said steering roller including a sliding member provided non-rotatably at a position adjacent to each of ends of said steering roller with respect to a rotational axis direction, said sliding member being tiltable together with said steering roller and slidable with an inner surface of said belt; a first stretching roller configured to stretch said endless belt, wherein said first stretching roller is provided upstream of the primary transfer portion (T1Y, T1M, T1C, T1K) and downstream of a contact portion between said tiltable steering roller and said endless belt with respect to a rotational direction of said endless belt and is provided rotatably with play with respect to a rotational axis direction of said first stretching roller; and a second stretching roller configured to stretch said endless belt, wherein said second stretching roller is provided upstream of the secondary transfer portion and downstream of the primary transfer portion (T1Y, T1M, T1C, T1K) with respect to the rotational direction of said endless belt and is provided rotatably with play with respect to a rotational axis direction of said second stretching roller, characterized in that said first stretching roller is smaller in outer diameter than said second stretching roller, with respect to the widthwise direction of said endless belt, a width of a first contact portion between said first stretching roller and said endless belt is larger than a width of a second contact portion between said second stretching roller and said endless belt, and said first stretching roller is disposed at a position where end portions of the first contact portion are nonoverlapping with end portions of the second contact portion due to the play of the first and second stretching rollers (4 and 5). an image forming portion (PY, PM, PC, PK) configured to form a toner image; an endless belt which is provided opposed to said image forming portion (PY, PM, PC, PK) and onto which the toner image formed on said image forming portion (PY, PM, PC, PK) is transferred; a primary transfer roller (15Y, 15M, 15C, 15K) provided opposed to said image forming portion (PY, PM, PC, PK) through said endless belt and configured to form a primary transfer portion (T1Y, T1M, T1C, T1K) for transferring the toner image formed on said image forming portion (PY, PM, PC, PK); a secondary transfer roller contacting an outer surface of said endless belt and configured to transfer the toner image from said endless belt onto a recording material at a secondary transfer portion; a tiltable steering roller configured to adjust a position of said endless belt with respect to a widthwise direction, said steering roller including a sliding member provided non-rotatably at a position adjacent to each of ends of said steering roller with respect to a rotational axis direction, said sliding member being tiltable together with said steering roller and slidable with an inner surface of said belt; a first stretching roller configured to stretch said endless belt, wherein said first stretching roller is provided upstream of the primary transfer portion (T1Y, T1M, T1C, T1K) and downstream of a contact portion between said tiltable steering roller and said endless belt with respect to a rotational direction of said endless belt and is provided rotatably with play with respect to a rotational axis direction of said first stretching roller; and a second stretching roller configured to stretch said endless belt, wherein said second stretching roller is provided upstream of the secondary transfer portion and downstream of the primary transfer portion (T1Y, T1M, T1C, T1K) with respect to the rotational direction of said endless belt and is provided rotatably with play with respect to a rotational axis direction of said second stretching roller, characterized in that said first stretching roller is smaller in outer diameter than said second stretching roller, with respect to the widthwise direction of said endless belt, a width of a first contact portion between said first stretching roller and said endless belt is larger than a width of a second contact portion between said second stretching roller and said endless belt, and said first stretching roller is disposed at a position where end portions of the first contact portion are nonoverlapping with end portions of the second contact portion due to the play of the first and second stretching rollers (4 and 5).