A stretched polyester film and a method for preparing the stretched polyester film are disclosed. The stretched polyester film contains a polyester resin in which a diol component and a dicarboxylic acid component are polymerized, wherein the heat shrinkage rate of the film at 80 C. in the main shrinkage direction is 40% or more, and, upon a first thermal treatment and a second thermal treatment, a first melting temperature (T.sub.m1) and a second melting temperature (T.sub.m2) appear when the film is analyzed by differential scanning calorimetry (DSC). The stretched polyester film has excellent heat shrinkability, along with an increased efficiency of a recycling process of waste plastic containers.

A procedure wherein a tubular high molecular weight polymer film which has been longitudinally stretched and therefore is longitudinally shrinkable, is converted by helical cutting to second polymer film, which then is heated and relaxed in order to partly or totally eliminate the shrinkability, whereby one edge of the helically cut film, inevitably becomes longer than the other edge, referring to a relaxed stated, characterised in that second film is converted to third film by continuously advancing second film in a first direction towards a lineary zone which extends perpendicularly to the edges of the cut film, while heating the film to a temperature lower than but close to its melting range, and in direct succession hereto moving the heated film at velocity (v) in a second direction which forms a small angle (a) to the first direction, the velocity (v) and angle (a) being selected to reduce or totally eliminate the difference between the lengths of the edges.

Provided is a method capable of producing a retardation film having an elongated shape and having a slow axis in an oblique direction with high production efficiency while precisely controlling the direction of the slow axis and the variation of the direction. The method of producing a retardation film of the present invention includes: holding the left and right end portions of a film with left and right variable pitch-type clips configured to have clip pitches changing in a longitudinal direction, respectively; preheating the film; obliquely stretching the film; and releasing the clips holding the film. The oblique stretching includes making the change ratio of the clip pitch of the clips on one side in the traveling direction thereof and the change ratio of the clip pitch of the clips on the other side in the traveling direction thereof different from each other, and/or making the position at which the clip pitch of the clips on the one side in the traveling direction starts to change and the position at which the clip pitch of the clips on the other side in the traveling direction starts to change different from each other, while extending a distance between the left and right clips. In the present invention, the moving speeds of the clips are braked immediately before the holding step and/or during a time period from the holding step to the oblique stretching step.