A dichroic-mirror array in which a plurality of dichroic mirrors M, M, ···, M are arranged in a numerical order in a first direction with N ≥ 2, the first direction corresponding to an arrangement axis, wherein each of normal vectors on front surfaces of the plurality of dichroic mirrors is configured by a sum of a positive component in the first direction and a negative component in a second direction perpendicular to the first direction, the second direction corresponding to an emission axis, both the second direction and the emission axis being parallel to and oriented in a direction along which a light beam is incident on the dichroic mirror array at dichroic mirror M, wherein the normal vectors are substantially parallel to each other, and wherein, with 0 ≤ θ 0 ≤ 90°, when an average of angles which are formed by the normal vectors and a direction opposite to the second direction is referred to as θ 0, an average of refractive indices of substrates of the dichroic mirrors is referred to as n 0, an average of widths of the substrates of the dichroic mirrors is referred to as α, an average of thicknesses of the substrates of the dichroic mirrors is referred to as β, and an average of intervals between the dichroic mirrors along the arrangement axis is referred to as x, wherein a lower end of the dichroic mirror M is disposed to be shifted upward in a negative direction of the emission axis by a step difference y with respect to a lower end of the dichroic mirror M, wherein a lower end of the dichroic mirror M(n) is disposed to be shifted upward in a negative direction of the emission axis by a step difference z with respect to the lower end of the dichroic mirror M(n-1) with 3 ≤ n ≤ N, wherein θ 0, n 0, α, β, x, y and z satisfy a predetermined relationship so that the step differences y and z and the average interval x are optimized according to the average width α, the average reflective index n 0 and the average thickness β of the dichroic mirrors so as to increase an aperture width of the dichroic-mirror array while minimizing the optical path length thereof by taking into account the increasing influence of a ratio of β to α and a ratio of β to x on the aperture width of the dichroic-mirror array. wherein each of normal vectors on front surfaces of the plurality of dichroic mirrors is configured by a sum of a positive component in the first direction and a negative component in a second direction perpendicular to the first direction, the second direction corresponding to an emission axis, both the second direction and the emission axis being parallel to and oriented in a direction along which a light beam is incident on the dichroic mirror array at dichroic mirror M, wherein the normal vectors are substantially parallel to each other, and wherein, with 0 ≤ θ 0 ≤ 90°, when an average of angles which are formed by the normal vectors and a direction opposite to the second direction is referred to as θ 0, an average of refractive indices of substrates of the dichroic mirrors is referred to as n 0, an average of widths of the substrates of the dichroic mirrors is referred to as α, an average of thicknesses of the substrates of the dichroic mirrors is referred to as β, and an average of intervals between the dichroic mirrors along the arrangement axis is referred to as x,