The minimal representation π of the indefinite orthogonal group O(m+1, 2) is realized on the Hilbert space of square integrable functions on ℝm with respect to the measure |x|−1dx1 ⋯ dxm. This article gives an explicit integral formula for the holomorphic extension of π to a holomorphic semigroup of O(m+3, ℂ) by means of the Bessel function. Taking its ‘boundary value’, we also find the integral kernel of the ‘inversion operator’ corresponding to the inversion element on the Minkowski space ℝ m, 1.