Millimeter-wave absorption spectroscopy combined with a pulsed-jet expansion technique was applied to measure the internal-rotation band of He-HCN in the frequency region of 95-125 GHz. In total 13 rovibrational lines, split into nitrogen nuclear hyperfine structure, were observed for the fundamental internal-rotation band, j=1-0. The observed transition frequencies were analyzed including their hyperfine splitting to yield an intermolecular potential energy surface, as improved from the one given by a coupled-cluster single double (triple) ab initio calculation. The surface obtained has a global minimum in the linear configuration (He...H-C-N) with a well depth of 30.2 cm(-1), and a saddle point located in the antilinear configuration (H-C-N...He) which is higher by 8.91 cm(-1) in energy than the global minimum. The distance R-m between the He atom and the center of mass of HCN along the minimum energy path shows a strong angular dependence; R-m is 4.169 and 4.040 A in the linear and antilinear forms, respectively, while it is 3.528 A in a T-shaped configuration. In the first excited internal-rotation state (j=1), levels with l less than 4 are bound but not for the one with l = 5, according to the energy level diagram calculated from the present potential energy surface, where l denotes the quantum number for the end-over-end rotation of the complex. The energy level diagram is consistent with the millimeter-wave observation, in which the Deltal=0 transitions with l=0similar to4 were observed but not for those with l equal to or greater than 5. The band origin of the internal-rotation band, 98.70 GHz, as defined to be the same as the frequency of the R(0) transition, is larger by 11% than the J=1-0 rotational transition frequency of the free HCN molecule.