PURPOSE: To promote ventilation of gas, generated at the interior of a counter weight and drying of an internal filler by a method wherein an inlet and an outlet for air are formed in proper positions in the counterweight, and the flow of air blowing through the counterweight is formed during running of an engine.

CONSTITUTION: A part of cooling air to an engine 2 is caused to flow through a draft hole 15, formed right before a radiator 3, to a counterweight 1. Cooling air flowing to the counterweight 1 promotes drying of a filter 12 and is discharged through a draft hole 16 formed in a rear position. This constitution always purges gas internally generated by the filler 12 in the counterweight 1, and prevents a risk of ignition, in a work wherein fired is used.