An electrochemical reaction unit including a unit cell, a cathode-side member, and an anode-side member. The sum La of the distance Lai between a virtual straight line representing a center position of the unit cell and the midpoint between opposite end points of a cathode-side supply opening group and the distance Lao between the virtual straight line and the midpoint between opposite end points of a cathode-side discharge opening group is smaller than the sum Lf of the distance Lfi between the virtual straight line and the midpoint between opposite end points of an anode-side supply opening group including an opening of an anode-side supply communication channel and the distance Lfo between the virtual straight line and the midpoint between opposite end points of an anode-side discharge opening group including an opening of an anode-side discharge communication channel.