In a low-frequency sound reproducing speaker apparatus of Kelton type, a cabinet for containing a speaker unit is constituted by a shape of substantially a parallelepiped slender in a front and rear direction, a length dimension in the front and rear direction is set to a value substantially near to a depth dimension of a seat cushion of a vehicle, the speaker unit is attached to a partition plate for partitioning inside of the cabinet into front and rear air chambers, and a lower face of the cabinet is fixed to the seat cushion by a fixing belt laid along the front and rear direction.