To provide a check valve structure made of a sheet, can surely preventing a backflow and reducing resistance of a fluid flow (following current) from an upstream side to a downstream one, and to achieve miniaturization of a check valve.

Front and rear valve sheets 16 and 17 are joined together with right value seal 33 and left valve seal 32 elongated in a vertical direction, and a check valve channel 21 is defined by the right and left valve seals 33 and 32. A part between upper ends of the right and left valve seals 33 and 32 serves as an upstream port 22, and a part between lower ends of them serves as a downstream port 23. The front valve sheet 16 is elongated above the upstream port, and an elongated protruding part 19 thereof is joined to a front bag sheet 13 with a front elongated part seal 40. The rear valve sheet 17 includes a protruding part 20 elongated above the upstream port, and the protruding part 20 is joined to a rear bag sheet 14 with a rear elongated part seal 41.