To provide a straightener, whose measurement accuracy is high and which reduces the processing operation regarding a flow measurement is spite of being of a small size and to provide a gas meter.

A first mesh 15A and a second mesh 15B are arranged so as to sandwich straightening plates 14 on the way of a flow channel 53 in which a flow rate sensor 10 is arranged, and a third mesh 15C is arranged by keeping a prescribed interval on the upstream side from the first mesh 15A. The lengths of the straightening plate 14 before and after the flow rate sensor 10 are made mutually identical along the flow direction.