The discovery occurred last Saturday after security forces had to remove another explosive device found on a construction site in the city of Campo Grande.
A 10-year-old girl who was watching the operation noticed that the device uncovered by the police bore a striking similarity to the metal cylinder her parents used as a doorstop and sounded the alarm.
"She came home in desperation because the object was just like ours," a member of the girl's family said.
The device was found in 1997 when the family was digging an irrigation ditch on their land and they decided to keep it.
"We never imagined it could be a bomb - I kept it because it was different," she told the online news site G1.
Soldiers were expected to visit the family to remove the device and announce exactly what it is.
The military said they are determining whether a field in the area had once been used for army exercises.
Inside Photos: Gauri Khan, Shweta Bachchan, Aishwarya Rai go glam!
Photos: Malaika Arora spotted at a salon in Bandra
Inside Photos: Malaika Arora, Kareena Kapoor Khan party with Isha Ambani
Photos: Deepika Padukone with father Prakash at Mumbai airport
Sachin Tendulkar, son Arjun join Aaditya Thackeray for cleanliness drive in Bandra