Disclosed is a laminated heat exchanger wherein deterioration of sealing performance of a sealing portion of inlet and outlet header sections due to deformation of the sealing portion can be eliminated, said deformation being generated at the time of having a PTC heater in close contact between flat heat exchanging tubes by sandwiching and pressing the PTC heater between the tubes. A heat medium heating apparatus using the laminated heat exchanger, and an in-vehicle air-conditioning apparatus using the laminated heat exchanger are also disclosed. A laminated heat exchanger (18) is provided with: flat heat exchanging tubes (17), each of which is configured by bonding two molded plates (17A, 17B) that are respectively provided with outlet and inlet header sections (22, 23) on both the ends of a flat tube section; and a heat exchanger pressing member (16), which presses and brings the flat heat exchanging tubes (17) to be in close contact with each other, said flat heat exchanging tubes being laminated such that the inlet and outlet header sections (22, 23) communicate with each other through communicating holes (24, 25). With respect to the inlet and outlet header sections (22, 23), at least one of the input position (P1) of a pressing force applied by means of the heat exchanger pressing member (16), a position (P2) where the pressing force is transmitted to the bonding portion of the two molded plates, and a position (P3) where the pressing force is transmitted to the sealing portion between the inlet and outlet header sections is set at a position not on the same line (L) in the pressing force direction.