We study the existence and nonexistence of a Cauchy problem of the semilinear heat equationHere, N ≥ 1, p = 1 + 2/N and φ ∈ L 1 (R N ) is a possibly sign-changing initial function. Since N (p − 1)/2 = 1, the L 1 space is scale critical and this problem is known as a doubly critical case. It is known that a solution does not necessarily exist for every. In this paper we construct a local-in-time mild solution in L 1 (R N ) for φ ∈ X q if q ≥ N/2. We show that, for each 0 ≤ q < N/2, there is a nonnegative initial function φ 0 ∈ X q such that the problem has no nonnegative solution, using a necessary condition given by Baras-Pierre [Ann. Inst. H. Poincaré Anal. Non Linéaire 2 (1985), . Since X q ⊂ X N/2 (q ≥ N/2), X N/2 becomes a sharp integrability condition. We also prove a uniqueness in a certain set of functions which guarantees the uniqueness of the solution constructed by our method.