610 a door 670 is provided in a lower part of the main unit 600 permitting maintenance of the inside of the ink jet printer next the internal structure of the main unit 600 will be described referring to fig2 electrical components such as a control circuit 645 are located in the upper part of the main unit 600 circulation system control components such as a solenoid valve unit 650 and a pump unit 655 are housed in a rear lower part 680 of the main unit the solenoid valve unit 650 has a supply valve a purge valve and an exhaust valve an ink tank 1 is housed in a front lower part 660 of the main unit the door 670 can be opened and closed so that the ink tank 1 can be pulled out of the main unit 60 making maintenance including refilling and disposal of ink or solvent easy next details of the control circuit 645 will be described referring to fig3 a cpu 300 controls the ink jet printer according to the present invention a rom 310 is a read only memory which stores a program and control data required for operation of the cpu 300 a ram 305 is a rewritable memory which temporarily stores data which the cpu 300 deals with during execution of the program a bus line 380 is a signal line for all signals including data address.