A heat sink has a panel body and a heat radiating portion. The panel body includes a working fluid circuit for circulating a working fluid in the panel body and has a heat receiving portion and a heat radiating portion substantially on the same plane. The heat radiating portion has an upper region and a lower region. The heat receiving portion is provided on a lateral side of the lower region of the heat radiating portion, and a plurality of radiating fins are provided on at least one surface of the heat radiating portion of the panel body. The working fluid circuit extends from the heat receiving portion to the lower region of the heat radiating portion and further to the upper region of the heat radiating portion, and the working fluid circuit in the lower region of the heat radiating portion has an average channel cross sectional area which is smaller than an average channel cross sectional area of the working fluid circuit in the heat receiving portion.