A casing of a liquid-cooled-type cooling device has a peripheral wall including mutually facing right and left side walls. A cooling-liquid inlet is formed at one end of the right side wall, and a cooling-liquid outlet is formed at an end of the left side wall corresponding to the other end of the right side wall. A parallel-flow-channel section is provided within the casing to be located between the left and right side walls and between the cooling-liquid inlet and outlet and includes flow channels through which cooling liquid flows. Internal regions of the casing located upstream and downstream of the parallel-flow-channel section serve as inlet and outlet header sections, respectively. The cross-sectional area of the inlet header section reduces from the cooling-liquid inlet toward the left side wall. The outlet and inlet header sections are asymmetric in shape with respect to the width direction of the parallel-flow-channel section.