An electrochemical reaction unit comprising: a unit cell including an electrolyte layer , and a cathode and an anode which face each other in a first direction with the electrolyte layer intervening therebetween; a separator having a through hole surrounding portion which surrounds a through hole extending in the first direction and is bonded to a peripheral portion of the unit cell by means of a bonding member containing a brazing material, thereby separating an air chamber facing the cathode from a fuel chamber facing the anode ; and a glass sealing member containing glass and being in contact with both the surface of the separator and the surface of the unit cell , thereby sealing between the air chamber and the fuel chamber , the electrochemical reaction unit being characterized in that : the separator has: a first flat portion having the through hole surrounding portion ; a second flat portion ; and a connection portion having a portion whose position in the first direction is different from that of the first flat portion and the second flat portion and connecting the first flat portion and the second flat portion , wherein the following relations are satisfied: L > 3 and H ⋅ L ≥ 0.5 where L represents the length (mm) of an effective bonding portion of the bonding member in a second direction perpendicular to the first direction, the effective bonding portion overlapping with the separator and the unit cell in the first direction, and H represents the height (mm) of the connection portion in the first direction. a unit cell including an electrolyte layer , and a cathode and an anode which face each other in a first direction with the electrolyte layer intervening therebetween; a separator having a through hole surrounding portion which surrounds a through hole extending in the first direction and is bonded to a peripheral portion of the unit cell by means of a bonding member containing a brazing material, thereby separating an air chamber facing the cathode from a fuel chamber facing the anode ; and a glass sealing member containing glass and being in contact with both the surface of the separator and the surface of the unit cell , thereby sealing between the air chamber and the fuel chamber , the electrochemical reaction unit being characterized in that : the separator has: a first flat portion having the through hole surrounding portion ; a second flat portion ; and a connection portion having a portion whose position in the first direction is different from that of the first flat portion and the second flat portion and connecting the first flat portion and the second flat portion , wherein the following relations are satisfied: L > 3 and H ⋅ L ≥ 0.5 where L represents the length (mm) of an effective bonding portion of the bonding member in a second direction perpendicular to the first direction, the effective bonding portion overlapping with the separator and the unit cell in the first direction, and H represents the height (mm) of the connection portion in the first direction. the separator has: a first flat portion having the through hole surrounding portion ; a second flat portion ; and a connection portion having a portion whose position in the first direction is different from that of the first flat portion and the second flat portion and connecting the first flat portion and the second flat portion , wherein the following relations are satisfied: L > 3 and H ⋅ L ≥ 0.5 where L represents the length (mm) of an effective bonding portion of the bonding member in a second direction perpendicular to the first direction, the effective bonding portion overlapping with the separator and the unit cell in the first direction, and H represents the height (mm) of the connection portion in the first direction. <EOS>