A flexible tube for use in an endoscope comprises a flexible basic tubular core structure including an outer meshwork tube, and a thermoplastic synthetic resin tube bonded to the outer meshwork tube of the tubular core structure in a manner such that the resin tube maintains an inherent compressive pre-stress in the finished flexible tube. An endoscope tube having flexibility which varies in a step-wise manner from one end of the tube to the other is obtained by integrally bonding two or more thermoplastic synthetic resin tube sections formed of respective resin materials having different hardnesses to the outer surface of the tubular core structure to form a coating layer in an analogous manner. The ends of adjacent tube sections substantially abut each other and upon heating the tube sections to a temperature only slightly higher than a softening point but lower than a melting point of any one of them, the localized regions of the respective pairs of abutting tube section ends are heated to a temperature which is as close as possible to a melting point of one of the tube sections having a higher melting point than that of the other tube section, whereby the end of the other tube section having a lower melting point is fused with the end of the adjacent tube section, and after cooling of the tube sections, a continuous and integral tube is obtained without any joints or junctures.