To provide an oil filler plug for gear casings, capable of effectively preventing a lubricating oil from spouting out from an air vent while the original function of the air vent is maintained.

With this oil filler plug 10, in a gear casing 1 where the oil filler plug 10 provided with an air vent 15 is removably installed to an oil filler port 9 provided at the upper part of the gear casing 1 in which transmission gears 2, 3 are arranged-installed, a space part connected to the air vent 15 is provided inside the oil filler plug 10, and a spouting-out preventing body 16 consisting of a cylindrical body 17 having air-gap connected to the space part in the outer periphery is fitted into the space part.