To provide an inner helically grooved tube in which pressure loss can be reduced advantageously and condensation effect can be enhanced effectively.

In the inner surface of a tube, a large number of first grooves 12 are made to extend helically in the tube axis direction at a helix angle of 10-30° with respect to the tube axis, and 1-6 second grooves 14 extending in the tube axis direction are made per one round while intersecting the first grooves 12 with a groove width 1.3-8 times as large as the pitch of the first grooves 12 in the circumferential direction of tube.