The brown-and-white, perky Momo was one of 32 successful candidates out of 70 dogs, passing a search and rescue test by finding a person in five minutes after merely sniffing their cap.
"Any breed of dog can be entered to become a police dog in the search and rescue division," said a Nara police spokesman.
But he admitted that news a Chihuahua had been entered may still come as a surprise to many.
"It's quite unusual," he said.
Television footage showed the 7-year-old Momo bounding across grass or sitting proudly, long hair blowing in the breeze.
Momo will be used for rescue operations in case of disasters such as earthquakes, in the hope that she may be able to squeeze her tiny frame into places too narrow for more usual rescue dogs, which tend to be German Shepherds.
The public response to the news of Momo's selection took police by surprise, the spokesman said, adding: "The phone's been ringing all afternoon."
A spokeswoman for the Nara Police Department in western Japan announced that the seven-year old Momo passed a canine rescue test in November, 2010 and started its new job in January 2011.
According to her, the 3 kg (about 7 lbs) dog spent less than five minutes sniffing out and discovering the target (a scent of a human survivor) within a 100 m² area.
“It’s quite rare for us to have a Chihuahua working as a police dog,” the spokeswoman told the Japanese news.
“We would like Momo to work hard and take advantage of its small size,” an officer of Nara Police Department was said.
Named after a state in Mexico, Chihuahuas are among the smallest breed of dogs. Momo joins ranks with the likes of German shepherds as a member of a disaster rescue squad in Japan.
Zec 4:10 For who hath despised the day of small things?
David had a sling; Dorcas had a needle; Mary had a jar of ointment; Moses had a rod!
Do you have a talent? Dedicate it to God! Use your talent which may seem small to you to do extraordinary great things!