A hydraulic oil tank has a housing, a connecting member and a filtering device. The housing includes a main body, a storage compartment and a first through hole. The storage compartment is formed within the main body for containing a the hydraulic oil, and the first through hole is formed through the main body. The connecting member is disposed on the main body and covering the first through hole, the connecting member including a contact face having a planar shape. The filtering device is disposed within the storage compartment. The filtering device is connected to a part of the contact face exposed within the first through hole. The filtering device includes an overlapped portion overlapping with an inner edge of the first through hole in a plan view of the first through hole seen from inside of the storage compartment.