The pen is suitable for 3.4MM ink sac.
There's a pump that sucks up ink Product Feature:
Retractable Fountain Pen with nib size: 0.38mm/0.015inch 
Please allowed 1-3mm(0.04-0.12in) differences due to manual measurement,
Due to the difference between different monitors, the picture may not reflect the actual color of the item.
 
NOTICE: ONLY GREEN IS AVAILABLE
Point Type: Extra Fine Material: Metal
This package includes:  The fountain pen, 30ml ink ( blue or black) and pen case
Great value for Gift!!