Sheet processing apparatus and image forming system

A sheet processing apparatus comprising:

• a pair of pressing members that nip and press against a fold section of a sheet;
• an advancing-receding unit that causes the pair of pressing members to repeatedly press against the fold section by causing at least a first pressing member of the pair of pressing members to advance to and recede from a second pressing member of the pair of pressing members, wherein the advancing-receding unit varies a reception separation amount and a post-receding separation amount from each other and sets the post-receding separation amount to be smaller than the reception separation amount, the reception separation amount being a separation amount by which the two pressing members are separated from each other when the fold section is to be received between the pair of pressing members, the post-receding separation amount being a separation amount by which the two pressing members that have pressed against the fold section are separated from each other after receding from the fold section.