I ended up drawing the IN-12A nixie clock's mainboard schematic. In schematic, The "pcb header" is the header that connects the nixie board and the power board. Pin A0~F9 is the nixies' cathodes. ( AB : hour, CD : minute, EF : second ) And 5v, 12v, GND is from power board(I'll make it later) I checked the schematic twice, but let me know if you have a better idea or my circuit has a problem. :)

Parts in the schematics are likely to arrive pretty late because of overseas delivery. In the meantime, I will order a PCB board and design a circuit for a nixie board and a power board.