sensors s3 and s4 respectively which detect the sizes of the sheets stored are detected when the pressure development is to be performed the receiving sheet 5 is supplied to the pressure development unit 1 from the manual insertion tray 2 or the cassette 3 or 4 selected in response to an input from an operation panel 34 as shown in fig5 the image forming apparatus is controlled by a central processing unit cpu 31 which reads data from the sensors s1 s2 s3 s4 and the operation panel 34 in accordance with a program stored in a rom 32 and controls the main motor 19 clutch 35 and others on the basis of the data the clutch 35 when engaged transmits a driving force of the main motor 19 to the power transmission unit 18 of the eccentric cam 17 a ram 33 stores the amount of rotation of the eccentric cam 17 corresponding to the sheet sizes stored in a memory area m when the pressure is adjusted for the eccentric cam 17 the cpu 31 reads data from the memory area m and the clutch 35 is accordingly engaged or disengaged referring to fig1 the process of adjusting the pressure for the eccentric cam 17 will be described by operating the operation panel 34 the manual insertion tray 2 or the cassette 3 or 4 is selected then the amount of rotation of the eccentric cam 17 is set depending.