A method of manufacturing a liquid ejection head includes: defining a first imaginary reference line along the longitudinal direction of a base member and measuring the distances between the first imaginary reference line and at least two liquid supply port portions of a liquid supply port row, including defining a second imaginary reference line passing the liquid supply port portion at the shortest distance from the first imaginary reference line and the liquid supply port portion at the longest distance from the first imaginary reference line respectively from among the at least two liquid supply port portions and defining the second imaginary reference line between two imaginary lines parallel to the first imaginary reference line.