A hydraulic pump having a swash plate that can be installed obliquely inclined relative to a rotational shaft and a plurality of pistons whose axial displacement is defined by the swash plate and that rotate with the rotational shaft, comprising: the rotational shaft receiving input of torque that is output from an electric motor; a housing that rotatably supports the rotational shaft; a cylinder block that is installed inside the housing, provided with a plurality of cylinder chambers extending parallel to the rotational shaft, and rotates with the rotational shaft; the pistons being supported relative to the respective cylinder chambers in the cylinder block so as to be slidably displaceable along a direction parallel to the rotational shaft 26); the swash plate coming into contact with ends of the pistons and being rotatable around the rotational shaft with the pistons, and being rotatable about a rotation centerline that is obliquely inclined relative to the rotational shaft; a case that is installed inside the housing and pivotably supported relative to the housing; a bearing portion that holds the swash plate relative to the case so as to be rotatable about the rotation centerline; and a piston biasing mechanism that is supported relative to the cylinder block and includes a spring that biases the pistons toward the swash plate, wherein the pistons reciprocate in the cylinder chambers with rotation of the rotational shaft, whereby the pressure of oil sucked into the cylinder chambers is raised and thereafter the oil is discharged wherein the piston biasing mechanism includes: the spring being supported relative to the cylinder block, directly or via a pedestal portion held by the cylinder block; a retainer that is attached to the spring at an end portion of the spring on a side opposite the cylinder block; and a planar piston biasing plate that is pivotably held relative to the retainer, and transmits biasing force from the spring to bias the pistons toward the swash plate, wherein each of the pistons passes through a hole provided in the piston biasing plate, and can be engaged with the piston biasing plate on a side opposite the cylinder block. the rotational shaft receiving input of torque that is output from an electric motor; a housing that rotatably supports the rotational shaft; a cylinder block that is installed inside the housing, provided with a plurality of cylinder chambers extending parallel to the rotational shaft, and rotates with the rotational shaft; the pistons being supported relative to the respective cylinder chambers in the cylinder block so as to be slidably displaceable along a direction parallel to the rotational shaft 26); the swash plate coming into contact with ends of the pistons and being rotatable around the rotational shaft with the pistons, and being rotatable about a rotation centerline that is obliquely inclined relative to the rotational shaft; a case that is installed inside the housing and pivotably supported relative to the housing; a bearing portion that holds the swash plate relative to the case so as to be rotatable about the rotation centerline; and a piston biasing mechanism that is supported relative to the cylinder block and includes a spring that biases the pistons toward the swash plate, wherein the pistons reciprocate in the cylinder chambers with rotation of the rotational shaft, whereby the pressure of oil sucked into the cylinder chambers is raised and thereafter the oil is discharged wherein the piston biasing mechanism includes: the spring being supported relative to the cylinder block, directly or via a pedestal portion held by the cylinder block; a retainer that is attached to the spring at an end portion of the spring on a side opposite the cylinder block; and a planar piston biasing plate that is pivotably held relative to the retainer, and transmits biasing force from the spring to bias the pistons toward the swash plate, wherein each of the pistons passes through a hole provided in the piston biasing plate, and can be engaged with the piston biasing plate on a side opposite the cylinder block. the spring being supported relative to the cylinder block, directly or via a pedestal portion held by the cylinder block; a retainer that is attached to the spring at an end portion of the spring on a side opposite the cylinder block; and a planar piston biasing plate that is pivotably held relative to the retainer, and transmits biasing force from the spring to bias the pistons toward the swash plate, wherein each of the pistons passes through a hole provided in the piston biasing plate, and can be engaged with the piston biasing plate on a side opposite the cylinder block.