The present invention provides a component built-in wiring substrate and a manufacturing method thereof. A component built-in wiring substrate (10) which includes: a core substrate (11); a plate-shaped component (101); a resin filling portion (92); and a wiring stacking portion (31), wherein, when viewed from the core principal surface (12) side, the projected area of the mounting area (32) is larger than the projected area of the plate-shaped component (101) and the resin filling portion, and the plate-shaped component and the resin filling portion are positioned directly below the mounting area (23), and wherein a value of the coefficient of thermal expansion (CTE [alpha]2) for a temperature range that is equal to or higher than the glass transition temperature of the resin filling portion is set to be larger than a value of the coefficient of thermal expansion of the plate-shaped component and smaller than a value of the coefficient of thermal expansion of the core substrate for the subject temperature range.