To provide an on-board case for a pet capable of readily achieving the prevention of fouling in the interior of a car and preventing a risk on the driving of the vehicle or the pet itself.

This on-board case 1 for the pet comprises a box body 2 capable of being developed and assembled for housing the pet, a detachable member 12 for mutually fixing the bottom and lateral faces of the box body 2, a fixing auxiliary member 5 for checking the release of fixing with the detachable member 12, a mooring member 7 for mooring the pet in the box body 2 and a fixing member 3 for fixing the housing body 2 on a seat 50. The mooring member 7 is provided with plural fixing rings 7d, beltlike members 7a, connecting members 7b and connecting members 7c. The beltlike members 7a are set at a smaller length than that of diagonal lines of the base 2a. When the pet is moored from the back side with the two mooring members 7, the pet can be prevented from jumping out to the outside. When a first belt member 3a and a second belt member 3b of the fixing member 3 are stretched over the seat 50, the housing body 2 can be brought into close contact and fixed on the seat 50.