The invention offers an improvement in a process for start-up in the occasion of producing acrolein and acrylic acid by catalytically oxidizing propylene at vapor phase under high load conditions, the start-up meaning the step of increasing the propylene supply rate (loading) from the non-reacting condition to the prescribed reaction conditions. This process is characterized in that the propylene supply rate is increased in the start-up stage of the reaction until the prescribed composition of starting reactant gas and the flow rate of the starting reactant gas are obtained, while adjusting at least one of the reaction temperature, the composition of the starting reactant gas and flow rate of the starting reactant gas, so as to maintain the propylene conversion at not lower than 90 mol %, the maximum peak temperature of the catalyst layer in each reaction zone at no higher than 450° C., and the sum of each ΔT (maximum peak temperature of a catalyst layer—reaction temperature) at the catalyst layer in each of the reaction zones to be no more than 180° C., respectively. According to this process, the reaction speedily reaches the steady state (standard operating conditions) and a high acrolein and acrylic acid yield is stably achieved from the start of the reaction.