Owner Bryan Gibson takes pictures of the gnome by overflowing drains to highlight their "diabolical danger".
He has named the gnome "Cormy" after Cormac, the civil engineering arm of Cornwall Council.
The council said it carried out "a planned routine maintenance programme" and checked 250 "hotspots" after storms or heavy rainfall.
Mr Gibson, from Bodmin, had previously taken about 15,000 pictures of drains over a period of about four years.
He only came up with the idea of featuring Cormy - who cost Â£1.99 - in the pictures two weeks ago.
Mr Gibson regularly sends the pictures to the company.
He said: "I want to highlight the diabolical danger of the whole country's drainage system."
Mr Gibson said poor drainage and high levels of surface water could cause cars to aquaplane and drivers to lose control.
A spokesman for Cornwall Council said there was no "significant problem with blocked drains" but blockages would always occur.
A garden gnome is being used in a one-man photographic campaign to improve road drainage.