PURPOSE: To effectively excavate a wide tunnel with an upper face arched by providing a plural of excavating means, alternately shifted in a longitudinal direction, at the front end of an excavating machine body with a continuous arched shield.

CONSTITUTION: An excavating machine body 2 is provided with a shield 3 having a plural of sectionally circular openings overlapping each other in part, continuously arranged in an arch shape as one unit and mutually communicating inside. A plural of through-hard-rock tunnel excavating means 5 are provided at the front end of the excavating machine body 2, which are alternately shifted in a longitudinal direction, arranged in series to be moved in and out via a thrust jack 26 and furnished with hard-rock cutting discs 7 and cylindrical shields 6. An arched tunnel is excavated in a rock-bed by using the excavating means 5. At this time, as each excavating means 5 can be independently moved in and out, excavating speed adjusted according to the hardness of the rock-bed. After excavating, the excavated face of bedrock is held by the shield 3 of the excavating machine body 2 until an arched ceiling wall is built up.