Toner image fixing device

A toner image fixing device comprising first and second toner image fixing rollers rotatable to each other in a state that said rollers are pressed each other;

• and a heating source contained in one of said fixing rollers;
• the surfaces of said first and second fixing rollers being covered with elastic layers, respectively, and a recording medium bearing a non-fixed toner image being passed through a nip between said first and second rollers to thereby fix the non-fixed toner image on said recording medium, wherein surface strain .di-elect cons.h in the circumferential direction of the elastic layer of said first roller which comes in contact with the non-fixed toner image on said recording medium, and surface strain .di-elect cons.p in the circumferential direction of the elastic layer of said second fixing roller are defined by:
• .di-elect cons.h.gtoreq.0.0583 (.DELTA..di-elect cons.).sup.4 -0.6 (.DELTA..di-elect cons.).sup.3 +2.1917(.DELTA..di-elect cons.).sup.2 -3.65(.DELTA..di-elect cons.)+6
• where .di-elect cons.h<6%, .DELTA..di-elect cons..ltoreq.3% and .DELTA..di-elect cons.=.di-elect cons.h-.di-elect cons.p.