An apparatus for mounting planar components onto a pad for a seat is disclosed. The apparatus includes a lower die for allowing the pad to be carried thereon in a condition where a section of the pad onto which the planar components are to be mounted faces upward, a support frame for supporting the planar components, the support frame having both side portions, the support frame being configured as a handling jig which is separatably combined with the lower die means from an upward direction of the pad carried on the lower die means when the planar components are to be mounted onto the section of the pad, and an upper die means for causing the planar components supported by the support frame to be thermal press-bonded onto the section of the pad, the lower die having spaced apart guide pins for engaging the both side portions of the support frame and causing the support frame to be guided in a condition where the support frame is parallel relative to the section of the pad carried on the lower die, and hook arms for causing the support frame to be hooked with respect to the lower die.