The invention provides an image forming apparatus capable of improving the maintainability of a post-treatment unit arranged at the upper part of a paper discharging space in the device. The image forming apparatus includes a main housing, a post-processing unit including a pivotable processing tray, and a recording medium discharging outlet. The processing tray includes a lock member, urging member and lock releasing member. The lock member is supported movably between first and second positions. The lock member engages with an engaging hole at the first position and disengages from the engaging hole at the second position. The urging member urges the lock member towards the first position. The lock releasing member resists a force applied by the urging member so as to allow the lock member to travel to the second position. A recording medium depressing member is disposed within a pivotal range of the lock member. A first slope is provided on at least one of a contact portion of the lock member and a contact portion of the recording medium depressing member.