A stapleless binding device includes a stapleless binding unit that performs a stapleless binding operation to form irregularities in a sheet stack by making forward and backward movements, a moving unit that relatively moves the stapleless binding unit or the sheet stack in a direction along one side of the sheet stack, and a holding unit that holds the sheet stack during the backward movement of the stapleless binding unit. The holding unit holds the sheet stack during the backward movement of the stapleless binding unit before the moving unit moves the stapleless binding unit or the sheet stack.