A heat exchanger, comprising: a first layer including a plurality of first flow channels being microchannels and arranged to extend side by side, a first one end-side collective flow channel being in fluid communication with one ends of the plurality of first flow channels, and a first other end-side collective flow channel being in fluid communication with the other ends of the plurality of first flow channels; and a second layer being laminated on the first layer and including a plurality of second flow channels being microchannels and arranged to extend side by side, a second one end-side collective flow channel being in fluid communication with one ends of the plurality of second flow channels, and a second other end-side collective flow channel being in fluid communication with the other ends of the plurality of second flow channels, wherein the first one end-side collective flow channel and the first other end-side collective flow channel include first microchannels A and B (15a, 15b), respectively, the first microchannels A and B (15a, 15b) extending in a direction perpendicularly crossing the direction in which the plurality of first flow channels extend and being constituted by grooves formed between ridges (14a, 14b) of rectangular cross section, gaps formed between neighboring ridges (14a, 14b) constituting first bypass flow channels A and B (16a, 16b) that extend in the direction in which the first flow channels extend, and the second one end-side collective flow channel and the second other end-side collective flow channel include second microchannels A and B (25a, 25b), respectively, the second microchannels A and B (25a, 25b) extending in a direction perpendicularly crossing the direction in which the second flow channels extend and being constituted by grooves formed between ridges (24a, 24b) of rectangular cross section, gaps formed between neighboring ridges (24a, 24b) constituting second bypass flow channels A and B (26a, 26b) that extend in the direction in which the second flow channels extend, wherein the first microchannels A and B (15a, 15b) are provided to extend parallel to each other, adjacent ones of the first microchannels A are in fluid communication with each other via the first bypass flow channel A, and adjacent ones of the first microchannels B are in fluid communication with each other via the first bypass flow channel B, and the second microchannels A and B (25a, 25b) are provided to extend parallel to each other, adjacent ones of the second microchannels A are in fluid communication with each other via the second bypass flow channel A, and adjacent ones of the second microchannels B are in fluid communication with each other via the second bypass flow channel B. a first layer including a plurality of first flow channels being microchannels and arranged to extend side by side, a first one end-side collective flow channel being in fluid communication with one ends of the plurality of first flow channels, and a first other end-side collective flow channel being in fluid communication with the other ends of the plurality of first flow channels; and a second layer being laminated on the first layer and including a plurality of second flow channels being microchannels and arranged to extend side by side, a second one end-side collective flow channel being in fluid communication with one ends of the plurality of second flow channels, and a second other end-side collective flow channel being in fluid communication with the other ends of the plurality of second flow channels, wherein the first one end-side collective flow channel and the first other end-side collective flow channel include first microchannels A and B (15a, 15b), respectively, the first microchannels A and B (15a, 15b) extending in a direction perpendicularly crossing the direction in which the plurality of first flow channels extend and being constituted by grooves formed between ridges (14a, 14b) of rectangular cross section, gaps formed between neighboring ridges (14a, 14b) constituting first bypass flow channels A and B (16a, 16b) that extend in the direction in which the first flow channels extend, and the second one end-side collective flow channel and the second other end-side collective flow channel include second microchannels A and B (25a, 25b), respectively, the second microchannels A and B (25a, 25b) extending in a direction perpendicularly crossing the direction in which the second flow channels extend and being constituted by grooves formed between ridges (24a, 24b) of rectangular cross section, gaps formed between neighboring ridges (24a, 24b) constituting second bypass flow channels A and B (26a, 26b) that extend in the direction in which the second flow channels extend, wherein the first microchannels A and B (15a, 15b) are provided to extend parallel to each other, adjacent ones of the first microchannels A are in fluid communication with each other via the first bypass flow channel A, and adjacent ones of the first microchannels B are in fluid communication with each other via the first bypass flow channel B, and the second microchannels A and B (25a, 25b) are provided to extend parallel to each other, adjacent ones of the second microchannels A are in fluid communication with each other via the second bypass flow channel A, and adjacent ones of the second microchannels B are in fluid communication with each other via the second bypass flow channel B.