The theory of the psychological tax contract suggests that tax authorities can enhance tax morale of intrinsically motivated taxpayers by respecting them. Testing this experimentally faces the difficulty of becoming an incompetent auditor. Given that tax evasion is always intentional in the standard tax morale experiments, in which a precise income level is shown to subjects, approving deceivers with respect could be a sign of a weak auditor, which creates a motive to evade, according to the ＂slippery slope framework＂. The results then contradict to the prediction of the psychological tax contract due to incorporating respect into experiments inappropriately. To overcome this problem, we create inadvertent tax evasion by showing subjects several income components for only ten seconds. Consistent with the theory, we find that tax morale was higher in the respectful treatment. This effect disappeared for subjects who could calculate total income within ten seconds. They might think that experimenters could not identify their purposeful evasion. Hence, their tax morale could only be raised by treating them harshly.