A needleless syringe for injecting an injection objective substance into an injection target area of a living body without using any injection needle, the needleless syringe comprising: a main syringe body (2,4,20) which is a main body of the needleless syringe and which has, in the main body, a sliding movement passage connected to an opening provided on a front end surface (4a1) of the main body; a piston which is arranged slidably in the sliding movement passage and which has an accommodating recess which is formed at a front end surface of the piston for accommodating the injection objective substance so that the injection objective substance is releasable; and a driving unit which applies energy to the piston in order to allow the piston to slide toward the opening, wherein: the opening is arranged so that the opening adjoins the accommodating recess and the opening and the accommodating recess are overlapped, so that an end surface area of the accommodating recess is included in an opening area of the opening when the piston is at an abutment position at which the piston abuts against the opening and sliding movement thereof is inhibited in the sliding movement passage; the accommodating recess is formed at an end surface of the piston facing the opening so that the accommodating recess does not penetrate in an axial direction of the piston; and when the energy is applied by the driving unit to the piston disposed at the initial position, then the piston slides in the sliding movement passage to abut against the opening at the abutment position, and thus the injection objective substance accommodated in the accommodating recess is discharged via the opening; characterised in that the piston further comprises a recess shaped for fitting with a corresponding projection (4b1) provided at a predetermined position along the sliding movement passage so as to arrange the piston at an initial position separated by a predetermined distance from the opening in the sliding movement passage, wherein the piston is arranged at the initial position in accordance with a fitting relationship between the projection (4b1) and the recess. a main syringe body (2,4,20) which is a main body of the needleless syringe and which has, in the main body, a sliding movement passage connected to an opening provided on a front end surface (4a1) of the main body; a piston which is arranged slidably in the sliding movement passage and which has an accommodating recess which is formed at a front end surface of the piston for accommodating the injection objective substance so that the injection objective substance is releasable; and a driving unit which applies energy to the piston in order to allow the piston to slide toward the opening, wherein: the opening is arranged so that the opening adjoins the accommodating recess and the opening and the accommodating recess are overlapped, so that an end surface area of the accommodating recess is included in an opening area of the opening when the piston is at an abutment position at which the piston abuts against the opening and sliding movement thereof is inhibited in the sliding movement passage; the accommodating recess is formed at an end surface of the piston facing the opening so that the accommodating recess does not penetrate in an axial direction of the piston; and when the energy is applied by the driving unit to the piston disposed at the initial position, then the piston slides in the sliding movement passage to abut against the opening at the abutment position, and thus the injection objective substance accommodated in the accommodating recess is discharged via the opening; the opening is arranged so that the opening adjoins the accommodating recess and the opening and the accommodating recess are overlapped, so that an end surface area of the accommodating recess is included in an opening area of the opening when the piston is at an abutment position at which the piston abuts against the opening and sliding movement thereof is inhibited in the sliding movement passage; the accommodating recess is formed at an end surface of the piston facing the opening so that the accommodating recess does not penetrate in an axial direction of the piston; and when the energy is applied by the driving unit to the piston disposed at the initial position, then the piston slides in the sliding movement passage to abut against the opening at the abutment position, and thus the injection objective substance accommodated in the accommodating recess is discharged via the opening; characterised in that the piston further comprises a recess shaped for fitting with a corresponding projection (4b1) provided at a predetermined position along the sliding movement passage so as to arrange the piston at an initial position separated by a predetermined distance from the opening in the sliding movement passage, wherein the piston is arranged at the initial position in accordance with a fitting relationship between the projection (4b1) and the recess.